Have played 4E once a week for about a year, and during that time played every level from 1 up to now 19.
And I really, really, really hate it.
The big problem is that the DM is a very good friend of mine, and we tend to have heated arguments about twice a week, which tends to put a lot of strain on things.
For that DM's sake, I've looked at every sourcebook I could lay my hand on, every Dragon article he has downloaded through his subscription, and followed lots of threads here on Enworld in hope of finding anything to like about the system so that I can ignore the things that bug me, but I just... cannot... find... anything.
I hate the combat-centricness, hate the lack of uses for non-combat magic, non-combat skills, the lack of non-combat powers. I've collected rituals by the bucket-load and gotten a bunch of skills, and I never get any-frickin'-use out of them. I hate how the game is only about playing with builds in the character editor for max DPS and then test-driving them...
I hate about how magic is just about bolting things with damage, or making one-turn no-flavor damage buffs.
I dislike the incredible narrowness, and well, boringness of magic items. They really feel like more of a burden than a reward.
I dislike how they have sacrificed a lot of the fun stuff "for balance", and then
failed to balance the combat system; In a team of fighter-rogue-swordmage-bard, the fighter routinely stands for 40-45% of the team's total damage (yes, we kept statistics), has the best defenses and hit points, and through his suck-all-monsters-in-and-keep-them-there powers has the best battlefield control, and through his damage aura stances (Rain of Steel) has the best area damage. That could have been ignorable, if there were anything else you could do that wasn't about damage, but there isn't anymore.