Well, 'something' is wrong with every edition of D&D
Myself, I had a lot of fun playing 3e until we reached ~ level 13, then the flaws of 3e really became obvious. As a DM prep time is very high and combats are painfully slow (it can take over an hour to play through a single turn!).
So, I'm going to be quite happy when we finally make the switch to 4e and I suspect I won't look back. We've playtested 4e for two weeks (playing every day) until about level 4 and quite frankly are having more fun playing it. Even the 4e sceptics in our group agree that it's fun.
What is difficult to grasp or rather difficult to come to terms with for some is the disjunction of pc mechanics and monster mechanics and the rather blatant 'gamist' approach to some things, e.g. the minion concept.
E.g. one of my players asked me how an npc necromancer was able to create an army of undead or how a npc mindbender would be able to dominate a large number of people for an extended time in 4e, since there are no rules for these things.
For me that's precisely the beauty of DMing 4e. Rather than digging through a dozen books trying to find the spells and feats that would allow a 3e npc to do these things, all I have to do in 4e is to decide that, yes, the npc can do this.
As long as I respect the monster math and encounter design guidelines, monsters and npcs can do anything I want them to. It's both creative freedom and less work. I can spend my prep time thinking about story and roleplaying aspects rather than complicated stat-blocks.
As much as I enjoyed tweaking monsters and npcs in 3e, I don't miss the investment of time I had to make.
For players, 4e can be a quite a culture shock, depending on what kind of classes you enjoyed playing in 3e. Here are some examples from my group:
One player would never play anything but a fighter in 3e. Now he's having the greatest time playing a wizard in 4e, because the class is now exactly as complex as any other class and the complexity is something he is comfortable with.
Another player who happened to be one of the 4e sceptics rarely played anything but spellcasters in 3e. He wasn't overly excited about the apparent lack of options in 4e since he enjoys complexity. What he does like, though, is the focus on positioning and group tactics in combat.
A third player was quite excited about more imortant role of skills (skill challenges), since she prefers the non-combat aspects and roleplaying in the game. She's also playing her pc more effective in 4e combat since she always struggled with the more complicated 3e rules.
Finally there's a player who is still rather unhappy that everyone else seems intent on switching to 4e. In 3e the game aspect he seemed to enjoy most was the character building / optimization. He was always stacking levels of all kinds of classes to create jacks-of-all trades. The multi-classing options in 4e just don't cut it for him. He still enjoys playing 4e but to him it's not 'D&D' anymore. It's more like the kind of fun he'd get out of an evening of board games.