@HentaiGod it's all about pacing, man. Axed mangas tend to shift into 12th gear all of a sudden, obviously rushing towards an ending while leaving every subplot either unresolved or give it a one-panel slapdash resolution. Obviously, that's pretty jarring and unsatisfying.
Dragging out is similarly unsatisfying, just in the other direction. Having buildup lead to nothing again and again, relying on tropes to prevent progress and preserve the status quo, etc. It's just a waste of a good story either way.
Basically, if a series paces itself well, and knows how long it wants to be from the start, it'll be great, short or long doesn't matter then. A series that gets cancelled or where the author suddenly realizes he can milk the premise for a lot longer is just frustrating (Amano Megumi comes to mind...).