I wished there had been more about the wedding, like all that with the wedding party moving closer together and the clock ticking was just lost. Artie and Quinn plot was all right. Artie's proving to be a good teacher. Sue being pregnant is a strange plot. Sue teaching Booty Camp was kind of odd as well, but perhaps she will make them better dancers.
Cooper was handsome, but not especially my type. Cooper overshadowing Blaine didn't thrill me. All in all, I would have preferred less of Cooper teaching bad acting and more sincere brotherly interaction.
This comes to a large extent from outside knowledge, but there was an in-canon acknowledgement of it, an episode where Blaine's hair was less gelled, "Blame It On the Alcohol." Blaine's hair is shown curly. "Young Blaine" has perfectly straight hair. There seemed to be no attempt at all to get a child who looked like a young Blaine would look. If they'd gotten a curly-haired child for the role, the question would have been why Blaine keeps his hair so heavily gelled, but that question is already being asked in canon.
(Later: I read various recaps and other online reactions, and a couple of bloggers pointed out that young Blaine's hair was gelled. Later still: If this is true -- I first saw it on the "big life, small space" Tumblr -- then I was right about Young Blaine's hair being straight, but wrong about what seemed like a lack of effort in casting Young Blaine: http://canadiantlc.tumblr.com/post/21197770130/klaineisnow-does-anybody-know-who-this-adorable)
I was expecting a lot, especially after a long hiatus. The "Somebody That I Used To Know" song was good. I guess we'll get more Blaine-heavy episodes in the fourth season.
(Added later: I appreciated the episode more when I watched it again.)