Stiff characters, wonky movements, lack of movement, consistent animation errors, etc. Actually, it's kind of hard to think of what constitutes as "bad animation". It's kind of just something that I know it when I see it.
Though I do want to say that just because a show has bad animation doesn't mean the cartoon itself is bad. This is usually the case, but then you have cartoons like Rocky & Bullwinkle, whose animation might as well be still images, but it's one of the greatest cartoons of all-time.