Iron Heroes was written by Mike Mearls under the Swords & Sorcery imprint that he shared with Monte Cook. Monte Cook would write Arcana Unearthed/Arcana Evolved under this line, which was more his style, but that's a completely different discussion.
Iron Heroes has a couple of big ideas, which I'll try to cover here:
This was their attempt to streamline the 3e skill system. Basically, skills would be grouped together, and then if you put a skill point into one, you'd get skill points in all of them together.
For example, the Armiger class has access to the Athletics skill group. If they put a point into Climb, they also gain 1 rank in Jump and Swim.
The Executioner class has access to the Athletics, Perception, and Stealth skill groups. If they put a point into Listen, they also gain 1 rank in Search, Sense Motive, and Spot. If they put a point into Hide, they also gain 1 rank in Move Silently.
If an Armiger puts a point into Hide, though, it only goes to Hide - they would need to increase their Move Silently skill separately.
Also, there are no more "cross-class" skills as defined by 3e. You put one point into a skill, and you gain one rank in it. What's supposed to represent cross-class-ness is whether or not you have access to the skill group to let you save on points.
Basically, take a penalty to a skill check to gain an additional benefit, or, in-combat, succeed at a skill check to gain a bonus, such as an attack roll or damage roll bonus. This technically is something that you could have already with standard 3e, the book just formalizes it as a thing.
It's supposed to dovetail with ...
Whenever the player does something "heroic", the DM should award them with Glory Points. If you choose to spend Glory Points before rolling for an attack, skill check, or saving throw, you add a d10 to your roll for every Glory Point you want to spend. If you choose to spend the Points after you roll your d20, you add a d6 for every Point instead.
This is supposed to encourage risky behaviors and heroic actions: you throw yourself into harm's way to earn Points, and then you use those points to pull of skill checks that would normally be impossible.
Iron Heroes has no spellcasters, and thus no healers. So Mearls took the Reserve Points variant rule from Unearthed Arcana and applied it here. You have a number of Reserve Points equal to your maximum HP. Whenever you are not engaged in strenuous physical activity, you can convert the Reserve Points into Hit Points. When you're sleeping/resting, you both heal your HP and recover your banked Reserve Points. It effectively gives you twice your maximum HP, but not within a fight.
I suspect, but have no way of proving, that the concept of 4th Edition's "Short Rest" and "healing surges" may have had their genesis here.
Defense Bonus and Armor as Damage Reduction
These are again variant rules from Unearthed Arcana that Mearls made into default assumptions for IH.
Armor as Damage Reduction means that wearing armor no longer increases your AC, but instead reduces the damage you take from successful hits. Except Mearls took it one step further and made it a random roll. Whereas plate armor would give you a flat DR 4/- in the 3e variant rule, you would instead have roll 1d8 for your plate armor's DR in IH.
And then, since you no longer have an AC bonus from armor, you simply have a Base Defense Bonus, similar to Base Attack Bonus, that takes the place of what your armor should be.
(it's also worth mentioning here that the Base Defense Bonus actually goes up to what your expected AC value would be not only from wearing armor, but also from the periodic upgrades of that armor's enhancement bonus, from +1 to +5. This is because there are supposed to be no magic items in IH.)
(it's also the case that Base Attack Bonuses are also higher, in order to account for this. A level 20 3e Fighter would have a final BAB of +20/+15/+10/+5, but a level 20 IH Archer would have a final BAB of +25/+20/+15/+10/+5. That extra +5 is from the +5 weapon that they should have had, had they been playing in a setting where magic is real)
This is a big change, and a big fiddly change in a game that's already full of fiddly bullshit.
Feats are divided into two categories: General and Mastery. If it's a General feat, then it's the same thing as normal 3e feats. You learn it, you get the effects. Improved Initiative is (still) a General feat. You learn it, you get a +4 bonus to initiative rolls.
If it's a Mastery feat, you can take it multiple times, and the benefit you gain is based on your Mastery Level. Let's try an example:
At level 1, a Berserker has a Power Feat Mastery of 1, and nothing else.
At level 5, a Berserker has a Power Feat Mastery of 3, and Armor Feat Mastery of 2, and a Feat Mastery of 1 for all other categories.
Foe Hammer is a Power Mastery feat.
As a level 1 Berserker, you can only take its Mastery 1 version - whenever you hit a target, the target must have a Fort save DC 10+half-damage-you-inflicted, or suffer a -2 injury penalty to attack rolls until the end of their next turn.
As a level 5 Berserker, you can take either its Mastery 1 version, or its Mastery 2 version - you may take a Full-Round Action to make a single attack. If it hits, the target must have a Fort save. If the target fails, its speed will be reduced by half.
As a level 11 Berserker, you can take the Mastery 1, 2, 4, or 6 versions of the feat. The Mastery 6 version will let you replace the a confirmed critical hit with inflicting the Sickened condition.
You have to wait until you get to level 15 Berserker to get access to the Mastery 8 version of Foe Hammer. But at level 15 Berserker your Armor Feat Mastery is only 7, and your Feat Mastery for everything else is only 6, so make sure you remember that!
But probably a bigger issue than the complicated mess of this thing is how the book manages to split up previously useful feats into buckets of warm spit.
Improved Trip: the first feat at Mastery 2 removes the OAs for Trip Attempts. The Mastery 3 feat gives you the +4 bonus in the opposed roll for the trip attempt. The Mastery 4 feat gives you the free attack after a successful trip attempt. So you need three feats and anywhere between character level 3 to 4 before you can get what you used to be able to have with just one feat from the 3e PHB.
The fact that IH gives you feats every odd level, and that there's a Mastery 6 upgrade of Improved Trip (target falls prone in adjacent square instead) and a Mastery 8 upgrade of Improved Trip (target must pass a Fort save or be stunned) doesn't really earn back all that you lose from this Feat Mastery system slicing up whole feats into mealy-mouthed quarter-feats.
I'm going to end this post here to give us something to chew on, because the individual class discussions are going to be much longer, but suffice it to say that the general rules framework that IH is built around is this creaky, unoriginal structure cobbled together from Unearthed Arcana variant rules that only serves to wreck all possible easy conversion to other d20 games.
There are two more big concepts in Iron Heroes that I forgot to mention:
Everyone is assumed to be human. Instead, every character has two "Traits", which provide certain bonuses that are supposed to take the place of whatever abilities you would have normally gained from having a race.
Traits are subdivided into Background, Mental, and Physical categories. Characters can only have two Traits max, but are limited to one Background Trait.
These are things like:
Artisan - Gain 4 ranks in Craft at level 1, and your Craft skill always increases by 1 rank each level. Raw materials for crafting cost 10% less, and you have a choice between [Taking 20 on Craft checks halves the time expenditure], [you deal 2x damage to items when trying to destroy them], or [you gain a +2 bonus to Appraise checks and can sell items for 10% more].
Bloodthirsty - when you kill an opponent, you gain a +1 bonus to damage and a +2 bonus to Will saves. There's a very pedantic clause attached to this that says the opponent had to have been killed from a positive HP score; that Coup de Graces don't count.
Desert-born - you get a +2 bonus to all saves versus Fire. When you are in the desert, you can always make Survival checks even when untrained, and you gain a +4 bonus on Survival checks. You only consume half as much food and water when you are in the desert. You also have a choice between [gain a +2 bonus to Balance and Tumble checks while walking on sand], or [when you are reduced to less than 0 HP, you can make a Fort save to not fall unconscious and continue to fight normally (Camel's Tenacity)]
There are Mental Traits that give you a +2 bonus to your Charisma or Intelligence or Wisdom.
There are Physical Traits that give you a +2 bonus to your Strength or Dexterity or Constitution.
However, these always come with a -2 penalty to something else, unless you take the Trait a second time to remove the penalty. But that already eats up both your Traits.
More Hit Points
Since this is a gritty, Conan-esque implied setting, characters need to be hardier. So instead of a Hit Die, characters now have a flat HP amount plus a rolled value.
A Thief in Iron Heroes, for example, has a Hit Die of [1d4+4], as opposed to a 3e Rogue's [d6]. So a Thief's worst HP roll is only going to be 1 less than a Rogue's best HP roll, and a Thief is going to have 2 more HP per level than the Rogue, on average.
I assume it's because, since there are no wizards in IH, the idea is to give players more "stuff" to buy and fiddle with to build their characters. Of course, this is an utterly ass way to do that, and betrays that base d20 non-casters are fucked yet going forward no one involved would try to undo this implicitly acknowledged problem.
Accurate Shot: For every 1 Aim Token you spend, you reduce the target's AC-bonus-from-cover by 2
Armor Piercing Shot: For every 1 Aim Token you spend, you ignore 1 point of the target's DR
Deadly Shot: Spend 2 Aim Tokens to deal extra damage equal to half your Dex modifier. You can only add this extra damage once per attack.
Disrupting Shot: When you hit the target, they must make a Fort save. On failure, they suffer a -1 penalty to their attack rolls for every 1 Aim Token spent, until the end of their next turn. This replaces all of the damage that you might inflict with that attack.
Distant Shot: For every 1 Aim Token you spend, you reduce the ranged penalty for your attacks by 2.
Hamstring Shot: Ready an action to shoot at your target when they move. When your readied attack hits, the target must make a Fort save, whose DC is increased by the number of Aim Tokens spent. If they fail, their movement speed is reduced to half until the end of their turn.
Storm of Arrows: As a Full-Round Action, fire a volley of arrows at a target. For every 2 Aim Tokens you spend, you fire 4 additional arrows. If your normal attack roll hits, then your target takes damage as normal. However, any other enemies adjacent to your target will also take damage if your attack roll was also high enough to beat their Defense. These adjacent enemies will take 1 damage per additional arrow fired (so a minimum of 4 damage by spending 2 Aim Tokens). The damage from these additional arrows is capped at your Dexterity modifier.
Unerring Shot: Before you shoot, your target must make a Reflex save, whose DC is increased by the number of Aim Tokens spent. If they fail, they lose their Base Defense Bonus against your next shot. This effect only lasts for your next shot.
While other warriors rely on speed or canny parries to defend themselves, the armiger trusts in his armor. Armigers create their own protective gear and improve it over time. They can stand before dozens of opponents, confident that their armor will shield them from harm.
The armiger wins battles by wearing down his foes. If you decide to play an armiger, keep close to your friends but stay near the front of every battle. Ideally, the most powerful monsters and opponents you face focus their attacks on you. This may sound suicidal, but the armiger excels at defending himself. Attacks that barely faze you could seriously injure your friends. As your foes’ attacks harmlessly rebound off your armor, you wear down their stamina and position yourself for a devastating counterattack.
Armored Trap: Spend 1 Armor Token to activate this ability, then ready an attack when you are hit by a melee attack. When you make that readied attack, you may spend additional Armor Tokens, gaining a +1 attack bonus for every Token spent this way. If you hit, you gain a damage bonus equal to the amount of damage absorbed by your armor from the attack that you just reacted to.
Combat Magnet: Activate this ability with a Full-Round Action and by spending 2 Armor Tokens. Whenever an adjacent ally is attacked, you may spend another Armor Token to force the attack to be resolved against you instead. This ability lasts until the end of your next turn. You cannot make attacks of opportunity while under the effect of this ability.
Defensive Stance: Activate this ability with a Full-Round Action. Whenever you are attacked, you may spend an Armor Token to make an attack of opportunity against your attacker.
Distracting Lure: Spend 4 Armor Tokens to force an enemy to make a Will save whenever they next attack you (and you may spend additional tokens to increase the save DC). If the target fails, they lose their Base Defense Bonus against the next attack against them.
Draining Defense: Spend 2 Armor Tokens and a Move Action to force an enemy to make a Will save. If they fail, the suffer non-lethal damage equal to 1d4 + your Constitution modifier.
Indomitable Wall of Iron: When you are attacked by an enemy, they must make a Will save, whose DC is increased by the number of Armor Tokens spent. If they fail, they suffer a -2 penalty to attacks against you, and a -2 penalty to their Defense (to the benefit of everyone attacking them).
Iron-Sheathed Counterattack: Spend 4 Armor Tokens to gain a free Trip or Disarm attempt the next time you are hit and the damage is completely absorbed by your armor's DR.
Sentinel’s Defense: Spend 4 Armor Tokens to let you roll and add your armor's DR value to your base attack checks, grapple checks, and strength checks.