Accel World, based on a light novel penned (I shit you not) by the same guy who wrote Sword Art Online. An anime so disappointing, so utterly devoid of wit, charm, and sincerity during its second half, that I found myself compelled to write a page-long rant/review in one of the usergroups here.
Upon further reflection, I think Accel World's problem is that it's a character-driven story, but the characters are all over the goddamn place. So inevitably, once the show jumped the shark, it came crashing down extra-hard.