While we're at it:
We Need A Healer. 4e made a step in the right direction in making many different kinds of healers, and healing not being an all-time-job, but even so, if your party lacks a healer, it seriously impacts your game. To the point that not having a healer is a pain the ass for everyone, so someone has to suck it up and play one.
What's In Your Hand Is Important. No other system I know makes such a big deal of "what you're holding right now"; D&D has a whole economy of actions just to switch what's in your hand. The Swordmage handjive is a travesty.
Combat Is It. Every edition, the lion's share of the rules are about combat. Sure, there are a few noncombat rules, but compare the robust options (feats, powers, spells, equipment) you have for combat vs. what you can do outside of combat. This is predicated by the notion that, to gain XP, you need to kill monsters. RP award Xp has never really had a codified system, nor achieving XP for non-combat goals; that's purely been up to the DM, and thus, rarely touched on in books with a system. Skill challenges are a move in the right direction, but other systems handle it way more elegantly where social situations are an actual mini-game, like combat is a mini-game.
Resource Management. That's really what D&D is about. Managing your loot to get better magical items. Managing your spells. Managing your actions. Managing your HP. I'm a very 'fast and loose and if it ain't relevant to the story I don't give a damn' type of person, so accounting for GP and XP for item crafting and what we want to sell to get GP to buy this, how much ammo I'm down or - AUGH. Book keeping is not fun for me at all.