Kill Me Baby. Granted I only watched two episodes before I dropped it, which is hardly half. I was expecting hilarity, and I got non-hilarity (Sure, the bear thing made me chuckle at first, but they drew it out far too long!)
Boku wa Tomodachi ga Sukunai- I was hoping the story could drown out the ecchi, but I got to the pool bit of episode two and Booberella's boobs just got annoying.
Those are the only two that are notable. I've never really dropped a series halfway in, it's normally right near the beginning or else I'm all "Well, I wasted this much time, I might as well finish it..."