Animu that disappoint me? Every Digimon season after the third one. Digimon was a substantial part of my childhood, and the fact that all the kiddies nowadays have crap Digimon animu to "enjoy" makes me sad. How will there be young Kaisers in the future if they don't have proper Digimon to be raised on?!
Gunbuster was an OVA series I had heard about for years and years. Two of my very good friends, who I respect a good deal, said it was among their favorite anime and I needed to watch it. One night, one of those friends and I marathoned the whole thing... at first I just thought I wasn't in the mood for some loldeep Gainax mess, but after watching it a second time I just couldn't take it. The tone was all over the place, like Anno didn't even know what kind of anime he was directing. The ending just didn't fit at all. It was just poorly paced, poorly made... ugh.
Also, Persona - trinity soul - was hella awful. I... well, there's just not much to be said on the matter. It was just bad. Astounding, since Persona 3 is essentialy the greatest thing to happen to video games since Super Mario Bros.
And yeah, Lucky Star comes to mind. It only took one episode to realize how bad it was and I wasn't really "disappointed", but I was sort of looking forward to it. I had heard great things about it on the Internet and from my friends in the Anime Club at college, but... wow, they spent like fifteen minutes talking about the proper way to eat their food. That's... not funny? It practically put me to sleep. People compare it to Seinfeld, which is just so wrong. Just because it's a show about nothing doesn't make it a groundbreaking comedy.