4e plays better than previous editions, in combat.
There are a few outstanding issues, such as battles dragging on and on when they are close ... which makes me miss the decisiveness of combat in 3.5e, but I guess when you design a game to have low lethality grinding becomes a possibility.
I do have some qualms about power creep, with each new book tipping the scales in favour of the PCs ... and item rarity had to be brought in as (effectively) a ret-con in order to rein in the madness.
(And no, it's not option creep, it's power creep. If your mid and high level characters' equipment and feats look increasingly similar, meaning there are fewer and fewer actual choices to make - such as the feat tax that was the expertise feats they added in 'cos the fundamental maths was borked, it's power creep)
Outside of combat, the mechanics of skill challenges are, for lack of a better word, ass.
4e played without skill challenges, where interactions are mostly free-form with an occasional die roll, functions IMO better than accumulating points for success or failure (regardless of how well it's masked; the mechanic itself is retarded).
The main issue I find a lot of people have with 4e is, it doesn't feel awesome.
See, the powers and combat grid pretty much tells you exactly what is happening, and it's often a case of using up your resources to underwhelming effect.
The vaunted game balance even limits your improvisation options to mechanics that deal equivalent damage as expected for your level.
So, at the end of the day it comes down to having slightly different methods of chipping away at your foes.
Which brings me to why wizards (among other things, wizards are just the easiest example) are OP.
'cos being able to manipulate the battlefield to an extent where daily powers can effectly be "I WIN" buttons, whilst having access to some of the highest damroll modifiers in the game .. which gets disgusting when used to multi-target ...
The game balance is actually not present if you're any good as an optimiser. And unlike previous editions, cracking down on charop in 4e equals telling your players that you would rather waste their time with lenghty unsatisfying grindy combat.