I find that so far, I'm really enjoying the questing and instancing more than the raiding in Lich King. Specifically:
• Naxxramas is very obviously a place made to support a lot more mobs than it now has, and the huge emptiness is oddly disconcerting after a while. I'm not sure what I'd suggest filling up some of the empty space with, but something would be good.
• Onyxia's Lair was and is a one-boss event, with 3-4 "trash" mobs to deal with before her. I liked that and still do. But I don't like quite so many other one-boss raids showing up. I find that it takes me a little while to get fully warmed up and engaged in a raid, so that I'm seldom at my best when it comes to Archavon or Obsidian Sanctum.
• Yesterday I took Spiderheart with friends on achievement-driven runs to Ahn'Qiraj 20 (complete clear), AQ40 (partial), and Blackwing Lair (complete), and it really brought home how far tactics have been stripped out. I certainly don't miss all of the fiddly bits. But I do miss engaging off-tanking, tanking in which AOE attacks are not the obviously best choice pretty much all the time, and like that.
I've read that the developers are intending to add more complexity in the big fights in Ulduar. I hope it works out!