Bleach. It started off with an interesting concept and then rapidly deteriorated into boring DBZ-style power battles and endless filler. I have no idea why it's so popular. It's not quite as mind-numbingly monotonous as DBZ, but it's pretty close.
Lucky Star, K-On! and Nichijou... though less nichijou because it has funny scenes it just takes too damn long to get to them. Also Kanon, good concept liked it the first two times I watched can't stand it now.
Trinity Blood. Dull as dirt, but really pretty art. The art is probably the only reason I still have it. That, and I got the box set for like, thirty bucks, which included the pretty art booklets.
Niea Under 7. Saw this a long time ago, so I'll probably give it another chance. Still, I remember being bored out of my skull watching however many episodes I managed to get through, which wasn't many.
Slayers. I know, this is probably a weird one. I was about twelve or thirteen at the time and for some reason just couldn't get into it. But like Niea, I'm gonna give it another chance sometime. Maybe I just wasn't in the mood to watch it.
Cowboy Bebop. Yes, THE GREATEST ANIME EVER bored me to freaking tears. I'm sorry Bebop fans, I really am, and I'm sincerely glad you enjoyed it. But I just couldn't bring myself to care about these characters and their shenanigans.