I like what I've seen so far. Granted, I don't have full acess, but what I have seen, and played through with the PHB Lite, makes me like it.
As a DM, the fact that exception based design takes me back to some of the good old days, of when I first began D&D is a good thing, as is ease of use. There are tons of examples of the 'universal' approach of 3.X D&D that are ridiculous, as anyone who hangs out in the charop forums can tell you. Monsters and players don't /need/ to use the same rules, and decoupling them, is, IMO, an improvement.
As a player, the fact that the fighter and the paladin now actually have tools to be 'sticky' or punish people for ignoring them to attack the casters is /great/. Before, unless the DM was playing the monsters as exceptionally stupid, the fighter had bo way to actually /protect/ the glass canons that were the spellcasters and rogues. And I like Tome of Battle, so it's nice to see a /similar/ mechanic (Not quite the same) that allows nonspellcasters to remain useful at higher levels. The system also seems less arcane, and much less of a cascading system. [By cascading, I mean this: An undead ambush results in a party member taking con damage, losing a level, ande becoming fatigued. You now have to change fort saves, carrying capacity, hit points, spells known, all saves, etc. By the time you actually figure out what just happened, half the players at the table have walked off. And then you realize you forgot that cat's grace was still up from the last encounter, so you wouldn't actually have been hit at all].
I am slightly worried about the loss of /mechanical/ differences between classes, but I have good roleplayers, and I expect that they'll behave quite differently, even with the same underlying mechanics. People who enjoy or are invested in the complexity won't like it, of course. The people who think nothing of throwing together a four or five PrC combination, if it gives them the flavor and abilities they like. And some people who enjoy playing commoners with swords are a bit upset by the assumption that PCs are heroes, and hence, special. I however, have always assumed so. If the world needs saving, there should be a reason why the party is needed. My PCs have always been heroes, and NPCs almost always lack the same sort of skills the PCs have. Chosen by fate, pawns of the gods, or what have you, they've always had second chances, action points, or whatever.
Ultimately, the new system, (so far) maintains the feel of D&D I love (even some from earlier editions) and ditches alot of the complexity. While there are a few things I was thinking I would miss, in retrospect, they were almost never used, or can still be done. I really don't /care/ if there's a detailed subset of the rules for grappling, or if the ogre just makes a fort save to prevent it. I don't care that undead use d12s and fey use d6s, because for a monster of the same CR, and same general role, the total HP are going to be almost exactly the same anyway. I don't feel a need to add two levels in 'aberration' to a beholder. I don't /care/ how many ranks in craft basketweaving my commoners have, after they max out profession: farmer. It /is/ a simpler system, especially in the individual actions. However, like chess or go, I think this new, 4e framework allows for nearly endless complexity in tactical considerations. Easy to learn, impossible to master. And of course, while I enjoy rolling the dice, I won't miss squeezing 12 seconds of combat into 2 hours, at any level past level 11. Or the fact that save or dies make doing damage utterly useless. Or well ... I could go on, but I think you get the idea.
Ultimately, I think I and my group will like 4e. I could be wrong. Or, I could just be a different audience than others. All I want out of a system is consistent resolution for chancey events. I play exalted, I like stunts in play. I play eberron, I like the PCs as above average individuals. I played modules, because I didn't have time to devote 3-5 hours of GM time for every hour of gameplay. I play with people who want their characters to do cool things, and change the world. For us? 4th Edition sounds like just what the doctor ordered.