Well my group and I have just ended our second foray into 4e this past Sunday with a unanimous vote not to go with 4e, that said I don't think 4e is the worst game evah, but it has some problems that don't mesh well with my particular group and our play style.
For me at least, it seems that everything I enjoy comes with a caveat that knocks it back into the con instead of pro realm for me. As an example I like that the stat blocks have been condensed, but do I feel like it makes the game easier to run... sure for people who think a certain way... people who are better at handling a lot of numerous but small scale details will thrive in running combats under 4e... those who are better at focusing on larger but less numerous tasks will probably have a better time running 3e... I'm of the latter part, as are my other players who run.
I mean one of them tried to run KotS (first, and probably last, time DM'ing) for us and told me later he just felt a little overwhelmed by how many things he had to keep track of and account for in running numerous monsters. So it wasn't the fact that the stat blocks were confusing, but that he would have had an easier time running with less opponents. The problem with 4e is that solos... and even some elites are the most boring combats in the new system (it ain't edition war, it's ATTRITION WAR!

), thus there is little to no support for a DM who is better at handling information in one way as opposed to another. Another final point about monsters is that in the new format it's hard for players to learn how best to combat a type of monster, since any experience they have is with one particular subset of that monster may or may not be applicable to the next subset... I guess for some monsters this makes sense, but for others I would rather the players learn from their encounters, I think this is a good type of system mastery.
I don't particularly care for the roles/power structure of classes... and for those who claim it's easier to design classes for I wonder... have you figured out whatever formula they used to determine the particular limits by role for such things as damage, movement powers, conditions, at-will vs. encounter vs. daily, etc.? Because I haven't. Also from play I don't know that classes are as balanced as they seem at first glance. In our party the defender routinely stepped all over the rogue and ranger's most damage shtick, by outputting more consistent damage than either of them... while still doing his defender thing. And, IMO, Wizards almost seem to suffer from the "rogue vs. undead" in 3e problem when faced with anything but minions, yeah they can help a little bit but really aren't equal to the other classes vs. everything else. Finally Warlords are the "depend on everyone else for my fun and effectiveness" class, even moreso than clerics in this edition... I just think any class that depends on others to be viable is bad game design, yeah it works in a really good gaming group, but not in one that isn't tactically coordinated.
This leads me to one of my biggest problems with 4e... the cranked up tactical combat feels like playing a separate wargame or a computer tactics game in the middle of our role-playing game and it breaks our immersion blatantly in the way it accomplishes this. I and my players feel immersion in our characters and their actions (whether simulationist or narrativist we don't care as long as it is stated) should have never been sacrificed for what basically amounts to a mini-game of chess every time a fight breaks out. We didn't get into D&D because we wanted to play wargames, we got into it because of the aspects that dealt with imagination and feel that part is being slowly subsumed under sound tactics, best movement positioning and optimal teamwork bonuses... I mean did Elric, Conan, Fafhrd and the Gray Mouser fight with the best tactics... no they did what was cool, fun and exciting to tell a story with and 4e (moreso than previous editions) seems to be sacrificing this.
There are more reasons, (the restrictiveness of character options, the value IMO that is not delivered in cost vs. material for the books, the wonky skill challenge rules having clarification articles only available to DDI subscribers, etc.) ...though but I feel this post has gone on long enough. In the end we have decided to stop D&D for awhile and play something else. Not sure what we'll be playing but when we do come back to D&D it will probably be 3.5.