Yeah, hong's got it right.
In short, Iron Heroes is a ruleset aimed around eliminating the need for PC magic, in the form of spells or items, while maintaining the same power level as D&D. Thus, the goal is that IH PCs can face CR-appropriate encounters at the same levels as D&D PCs, but whereas the D&D fighter may need a fancy magical wardrobe to do it, the IH fighter-equivalent just needs a decent suit of armor and a plain ol' sword. Additionally, magic is cast in the Howard/Leiber S&S mold of being the province of corrupted cult sorcerers, deranged immortals, sinister fiends, or weird Creatures from Beyond. (Thus, it's rather predictably NPC-geared.)
The advantages of this approach are twofold: First, it more closely simulates a wider variety of fantasy archetypes than does D&D. Conan, Fafhrd & the Grey Mouser, and for that matter Legolas and Gimli aren't really renowned for their stuff, but for their abilities. Second, it eliminates the need for tracking gear, magical buff effects, and all that stuff.
IH has its disadvantages too. First off, there are holes, although the
compiled errata and FAQ go a long way toward filling them. (In general, I'd argue that IH has far fewer holes even than D&D 3.5, especially when you throw in splatbook stuff.) Second, those who want not only lower magic, but a lower *power level* than D&D, will find that other systems (like Grim Tales) offer a better fit. Third, the game is not rules-light. I personally think that it's a lot easier than people give it credit for, but IMHO you have to experience the game to understand how it flows.
As to the major differences between IH and D&D:
1) The PC classes are almost all warrior variants: An archer, a heavy-armor guy, a berserker, a sneak-attacker, a movement-based light fighter, a master tactician, a jack-of-all-trades, a weapon master. The Thief (a class based around social skills and manipulation) and the optional Arcanist class are the two major exceptions.
2) Everyone gets a LOT of skills, and there are many more uses for them. There are skill challenges (take a penalty on a skill check to accomplish an unusual effect, like using Listen to pinpoint invisible opponents or Intimidate to demoralize several people at a time). There are giant feat chains built around getting more uses out of the skill. Some skill uses restricted to certain characters in D&D (bardic music) are for everyone in IH.
3) Feats are built into "feat masteries," which are basically feat chains with different prereq levels (all the way up to 10, which requires that your class be best suited to take the feat and that you be about 17th level). So, for example, Power Attack (1) works just like D&D Power Attack, but Power Attack (9) allows you to deliver an instant death attack if you take a -15 or greater penalty to rolls using PA.
4) Abilities are balanced not around uses per day, but around "tokens" which are generally per-encounter benefits. Only a few classes (and a few feats) grant tokens, and you pick them up by doing particular things (usually sacrificing actions or winning opposed checks). You track your token pool (we use poker chips or an index card; one player just writes them down next to his hp total), and in most cases, it goes away at the end of combat.
5) PCs get a bunch of abilities to shore them up against comparable-level D&D characters. Everyone gets two traits (similar to racial abilities) at 1st level, one feat every other level, a much more generous point buy than in core D&D, and better save bonuses (+1 per level to all saves). The game uses d4+x to generate hp (where x is anything between +2 for the casters to +8 for the barbarian types).
6) The game uses armor as DR and class defense bonus, as do several other d20 variants.
7) PCs have several combat options, some of which were introduced in Book of Iron Might and some of which are simply expanded classes of various actions that exist in core D&D. Besides skill challenges, there are combat challenges (like, say, fighting defensively in D&D, but with a field of similar options). There are stunts (basically opposed skill uses to generate combat benefits or to accomplish cool cinematic actions like blinding a basilisk for a round or firing an arrow into a dragon's maw to disrupt its breath weapon use). There are also zones (examples in D&D include traps, overturned furniture, and pretty much every kind of combat-usable terrain that module designers put into adventures; only here, there's a formal system of how to design them).
8) The NPC classes are *weak* compared to the PCs. NPC antagonist or "hero" types are generated using villain classes, which are basically a shorthand mechanism for whipping up in a hurry an NPC that fits a particular archetype (barbaric champion, dread sorcerer, demonic minion) without having to build the thing from scratch.
