I've mentioned this before, but two anime that definitely disappointed me were "Kampfer" and "K-on!" Season 2.
I'll not labour the point but as I mentioned previously many times, Kampfer was disappointing to the point where it almost made me angry. Dropped it after 2.5 episodes. So much wasted potential. Good seiyuu's, interesting premise, all ruined by one very stupid protag stuck in a plot exposition, misunderstanding, exacerbation feedback loop that eventually made me cringe so hard my eyebrows and forehead hurt.
K-on! season 2, as mentioned before, was especially disappointing because I loved the first season so much. So more K-on! should've been a good thing. Instead it bored me silly and I got very disappointed it didn't hold my attention as well as it should've.
Bonus joke answer! Rebuild of Evangelion 3.33 disappointed me because although I saw it in an arthouse cinema for a special screening and enjoyed it, it was disappointing not because the movie was bad but rather it took so bloody long to release it in the UK that I didn't bother waiting and instead bought a Hong Kong release, which had nicer binding on enclosed booklets and nicer outer packaging compared to the official UK release, which I also bought because apparently I'm a fool who is easily parted with his money when it comes to anime.
