All right, now we're getting to the fun part. I'll just start with an example of strategy: You've got a first-level character with a 12 in all ability scores and no particular special abilities beyond being trained in basic combat skills. Let's call him Jim. Jim is going to fight using a longsword and large shield. During your typical combat round, he can take a simple, effective approach: Spend a standard action to attack with his sword, a move action to make a block check with his shield and a swift action to threaten the area around him or take a five-foot step. Alternately, he could give up that guard check and make a full attack with his sword, which at first level means he'll score a second hit if his attack beats the opponent's AC by five.
Now, what if Jim was using a greatsword instead? Well, it would mean more damage, in the same fashion as 3.5 D&D. When making a full attack, he'd have to beat his opponent's AC by six rather than five to get that second hit in- hardly a deal-killer. No, the real drawback is when Jim wants to take that "guard and attack" approach he was using before- using the same weapon to attack and block in the same round gives him a -2 penalty on both checks.
Of course, there's still another option left to Jim besides defending with that greatsword: armor. For a starting character, a chain shirt is just as good for blocking as a light shield, with the added bonus of DR 2.
The Guard skill is the single biggest change to how combat plays out. Level, class and equipment all affect both your Reflex defense and your Guard skill in the same fashion, but guard gets an additional +5 bonus if it's a trained skill. Combine that with the fact that your guard skill doesn't decrease your defense on a bad enough roll, and characters are looking at an average of a +6 increase to defense if they take that move action. So even at first level, you've still got a set of meaningful choices: Guard and Attack to go for the middle ground, Full Attack for a chance to seriously increase the damage dealt if you exceed the opponent's defense enough, or Full Guard to make two Guard checks (against two opponents or a single opponent, taking the better result) in order to better defend yourself against an opponent. If Fighter A favors full attacks, Fighter B might take a full guard one round to better defend himself, then do a full attack the next round after Fighter A's full attack left him open. I'm considering letting characters make guard checks as immediate actions so long as they give up the appropriate action at the start of their action in the next round, which could disrupt a swift action-dependent special attack or other action.
So even at first level, characters have some simple, meaningful choices they can make before combat (two-handed weapon vs. one-handed and dagger/shield vs. other weapon setup that tends to be more specialized until you take the feats/abilities to properly support it) and during it (How much you should guard vs. how much you should attack). At higher levels the complexity increases as players gain new maneuvers and options from feats chains and, to a lesser degree, talents; tricks and stances that avoid universal increases and instead can be selected to personalize your fighting style as a whole.
To summarize: Mundane Combat in d20 rethought, compared D&D 3.5, should:
-Require less rolling. Though damage dice rolling potentially remains about the same, there are a maximum of two d20 rolls being made by each character each round
-Offer more fighting styles. The full attack with a greatsword is a viable melee combat option but not always the ideal one, even before artificial class restrictions and special bonuses are applied.
-Offer choices with uncertain outcomes on a round-by-round basis. Maybe you'll get lucky on that full atttack and inflict hordes of damage, while your opponent won't roll well enough to hit you even though you didn't guard. Or you could guard yourself, which on a 20 would make you nigh-impossible to hit and on a 1 would be no help at all. Score a critical success? Well, you can either take a quick, handy benefit or give it up for a shot at ending this combat much quicker.
-Offer increasing options over time. An accomplished high level fighter will have many more options at his disposal- the sword-and board warrior can force his shield up into the opponent's face and obscure their view as he hacks at a leg, or easily guard against multiple adjacent opponents while still being able to strike, and in a pinch can easily use only his shield to bash his enemy's face in.
I hope this all sounds like a viable way to improve the combat system.