We are several sessions into a mid-level D&D 4e campaign, and I don't hate it. Don't love it either, but I enjoy the group, and there are aspects of the game I do like and others that seem flat. None of it deeply offends me, though. But I do have some basic impressions that have developed with experience.
It is Both More and Less Complex Than 3.x
You heard me, I'm trying to have it both ways. The complexity of some combat rules are simpler to track (grappling, tripping, standing), the skill system is cleaner, the classes clearer and less easily turned into weird multiclass munchkin combos, and less likely to have one or two classes outshine other players in "screen time." There are fewer buffs and debuffs. But tracking marks and conditions takes a lot more work.
Combat is Long
I'm not convinced it's much longer than 3.x edition, and it sure isn't much worse than a typical Champions game, but none of those statements really does much to provide a good comparison. While increased hit points and slower, more steady increases in damage are part of the issue, so is the lack of save-or-die effects. While this does eliminate the anticlimactic 3.x dogpile on the one big monster tactic which will result in a one-quarter monster death, it can make simple fights drag out and do create less time for non-combat things.
Some of this is also DM encounter design. If I were running the game, there would probably be a bigger use of minions in large numbers with a few major monsters to create both interesting tactical dilemmas and opportunities for controllers and area affect attacks to shine. And to provide opportunities for badassery by PCs followed by having PCs handed their asses by non-minions. We seldom see minions, and often face big battles rather than several encounters over the course of a session.
Combat is Still Deadly
We have at least one person making death checks per game every game. As the party defender, that person is usually me.
Resource Management is Still There, It's Just Different
As a side note, this sort of encounter undermines the interesting but very different management issues of the game. Rather than managing spells, equipment and hit points as absolutes, play now involves managing per-encounter abilities and hit points; and daily powers, items, action points and healing surges. This makes a different tactical play style, but not alien to the earlier versions.
Teamwork is Key
And learning to synergize abilities, control and move across the battlefield, and use the team to set each other up to advantage is more closely integrated into the rules.
Defined Rules Lead to Ignoring the Really Cool Flexibility on DMG p. 42
The stunting rules and the page 42 table to figure out how to adjudicate them are one of the coolest things in the game. Getting my friends to see that is a challenge. It would be easy to blame them, but I think the very defined ruleset contributes a lot to the lack of desire to fudge things. This is very sad.
Hacking the Game isn't as Easy
Some parts are actually fairly easy to hack. New feats and races aren't so hard, though the tight balances assumed in the basic rules mean that finding mechanical ways to distinguish different races or feats may be hard to achieve. Coming up with new classes is downright hard. Coming up with a class means not only class abilities, but powers structured over 30 levels, including at least two paths in the class... which sounds more like work to me and less like fun.
In the end, it's a decent game. It's not crazy to see that it is an evolution of D&D: clearly so from 3.x, not so clearly so from the LBBs. It doesn't play like 2.x or later, but it plays well. All that said, I wouldn't run it without some major limits — it might be interesting to run a sword and sorcery game with only martial classes, a 10th level cap, humans only, and magic only in the form of rituals. But for now, I'll look elsewhere for games to DM.