From what I've gathered about the Soul Eater anime, it ran into the problem endemic to animated adaptations of manga: it caught up to the written story. When this happens, animation studios can do one of three things:
1: Go on hiatus until more story is written (not profitable, so a rarely taken option).
2: Make a filler arc/season until more story is written (most commonly seen in the big anime like One Piece, Naruto, Bleach, etc.).
3: Make up their own ending (arguably the worst possible choice because its almost never is as good as the manga's official ending).

I believe Soul Eater took the third option, which is why the ending was [s]bad[/s] abrupt, unsatisfying and overall more an end than a meaningful conclusion, like a crappy two-sentence conclusion lazily tacked onto the end of an essay (I hate writing conclusions, but I digress). They did the same thing with Fullmetal Alchemist too, which is why they made FMA: Brotherhood once the manga was finished. Regardless of its ending, I still liked Soul Eater.