I agree with mhacdebhandia, but I'll add a few thoughts of my own. On the one hand, 3e greatly simplified mechanics and made them more consistent. On the other, it added whole new layers of complexity. I played 2e for a while, and in a way I can't adequately describe, it had a certain flavor that 3e lacks, having replaced it with cold, hard logic. Now, I'm a programmer; cold hard logic has a special place in my heart, but I'd like to go back the other way a little. I can't cite anything specific that lost flavor, but the best I can say is that 2e felt more like "Here's what a role-playing world is like, and these are the rules used to describe it.", whereas 3e seems to take the approach "Here are the rules you use, and this is what you an do with them.". I'm not saying a good DM can't apply his own flavor and make a great game out of it, but I'm not a good DM; I'm a mediocre DM, and I'd like a little less technical manual and a little more top-to-bottom flavor.
As much as I revel in the rules-for-rules'-sake nitpicking over phrasing that appeals to my technical nature, I'd like to see a little more simplicity, a little less codifying, and a little more leeway. I understand the reasoning behind codifying things: to give consistency across DMs, in particular to reduce frustrations from arbitrary fiat of bad/inexperienced DMs because the rules have a definite answer. However, this also ties the DM's hand somewhat and makes it easier to get lost in minutiae. I'd like things to be a little bit looser and leave a little more room for interpretation, while simplifying things. I'm not saying undo everything, but put it somewhere in between. As examples:
- Having listen and spot separate gives you more granularity, but why not roll them into a single perception skill and hand-wave or apply a modifier on-the-fly in cases where it makes a difference?
- Do we really need to distinguish between animal, magical beast, and vermin? I found several dictionaries that list rats (animals in D&D) as example of vermin, and some even included coyotes and foxes. "Vermin" is sort of like the term "weed", but for animals: any animal that isn't wanted is a vermin. Should D&D have a weed type separate from plant?
- When magic is codified, it feels less, well, "magic" and more "technological" (and, again, I like technology). "Any sufficiently advanced system of magic is indistinguishable from technology." as the converse to Clarke's third law.
In some ways, I like d20 Modern's approach to classes better. I wouldn't import it wholesale into D&D, but I'd consider a similar approach, with fighter, wizard, rogue, and priest as the base classes, and all others as advanced classes. That may run contrary to what I just said, though, making the rules more elegant but removing more flavor; plus, what if I want to be a bard from level 1? I'll have to think on it longer.
I wouldn't complain if they replaced spell slots with MP either.