I'm in agreement with most people here on where I'd like 4e to go...
1) Make high level D&D playable/fun. I have to end all my games by level 13 or so, b/c above that, combat drags to a standstill. Between iterative attacks, Mass Dispels, stacks of buffs and effects causing everyone to recompute their rolls eternally, etc., it is currently a bit of a nightmare.
2) Love the Saga style reversal of Saves > Defense Values. Only 'active player' rolling dice.
3) Find some way to make Spell Resistance and Immunities a little less onerous, and more 'fun.' The 'all or nothing' approach to SR that we have currently just feels weird and wrong somehow, if there was a way to easily incorporate partial effects without more dicework, it would seem more realistic, and make high level combats less about wild swings of luck.
4) I like Armor as DR, and Reflex/Defense the same essential value. For lore creatures like werewolves, perhaps have a parenthetical, slightly lower Armor value for those who have Silvered Weapons.
...which brings me to...
5) End the Golfbag of situational weapons & mundane magical items. 3.5 make the 3.0 system of per + DR look archaic, but it did introduce the inane multiple weapon problem we have now. Magical weapons should be *magical*, and finding some way for them to scroll upwards in value automatically would be really sweet. I would love it if we could get back to the discovery of a magical sword being an epic event, one that the PCs would remember as their few relics grow and gain powers with them..
6) Change the naming convention for spell levels, tweak or abandon the Vancian system for something more flexible. Whether that's per encounter casting or spellpoints or whatever works out best.
7) Classes: Some Bard loving? They sucked in 3.0, and got upgraded to severely underpowered in 3.5. Bards are Love.
As far as classes go overall, I like the idea of a talent/feat system for extra customization: hundreds of different concepts should be doable with your PHB toolbox. If some crazy wizard wants to spend three of his feats to wear heavy armor and large shields, let him do so without giving up his class abilities...!
Multiclassing system that involves tradeoffs, but doesn't unduly punish spellcasters, monster classes, etc....
8) Less or no reliance on miniatures. Rules for minis are fine, but there has to be a way to make, for example, Sneak Attack useful (even if its implementation is slightly different!) in your head as well as on a grid.
9) Digital Initiative supports the DM and the players, doesn't enslave them.
10) Echo the grappling, turning, etc., complaints. Try to steamline or eliminate any rules subsystem that can currently only be used by looking up a table.