I can't even count the number of anime endings I've seen that I've been disappointed with, but the problem tends to lie in the fact that it's been based on an incomplete manga work that's still being published. So you wind up with the anime production team catching up, and throwing together some utterly mediocre closer, or worse, infinite amounts of filler while they try to pad out space for the manga to get ahead. Most recent disappointment of that sort I can think of would be the Soul Eater anime. Just a total nosedive when they got to the Arachnia arc compared to how it turned out in the manga, and just kept getting better afterwards. Though I suppose I'm happy they did just end it instead of pulling a naruto or bleach and just creating awful multi-season clumps of filler arcs.