Interestingly, I had difficulty recalling which anime disappointed me this year. If they were that bad, chances are I did not stick with them long enough for me to even remember them.
Here's my tentative list:
Another: For reasons described by others. It started really well, and I looked forward to a satisfying revelation and dramatic climax, only to be given an unexpected gore-fest. Even so, I wasn't exactly disappointed, because the ending was still somewhat entertaining, though not in the way I had hoped. Part of the fun, I think, came from observing how some forum members managed to put two and two together to figure out the mystery before the end.
Last Exile: Fam, the Silver Wing: By the mid-point, I stopped caring about the characters and the world of
Last Exile. The drama and action were contrived and unconvincing, but the visual elements were nice enough that I stayed with the series till somewhere near the end. Then I stopped because I was too busy and never went back to it. I loved the original series so, in comparison, its sequel was disappointing. But in truth, the story just never clicked sufficiently with me to register a genuine opinion.
Nisemonogatari: I think this was the one that came closest to being a true disappointment for me. I loved
Bakemonogatari and went into
Nisemonogatari with high expectations, only to be given pointless fan service and a barely meaningful plot. The disappointment may not have been as strong if I didn't subsequently try watching
Medaka Box as well. It had the same kind of self-indulgent, snarky humour that was fresh the first time round, but which quickly became wearying in the absence of characters I could truly sympathise with. My opinion of the stories' creater, NisiOisin, plummeted as a result.