1. Ripoffs of other animations.
There's such a thing as too much of a good thing - and while nothing is wholly original, making a CGI movie with green cat people and horrible motion-capture and calling it Gravatar will just make your audience groan and walk away. People like to see works in the same vein as other works they like, or even inspired by them. Direct copies clearly altered to barely avoid copyright infringement just look like blatant attempts to capitalize on a successful franchise with unoriginal pablum, and will usually come off as poor imitations.
2. Bad art and forgettable character design.
As the popularity of certain stick figure animations proves, you don't have to create great works of art to have a popular animation. But there's a difference between something that's deliberately bad or comically awful and something that's just plain shoddily and lazily done. Viewers will be able to tell the difference within thirty seconds, and once you've lost their interest, they won't bother with you again. Bland, forgettable characters with no thought put into their design will chase your audience away, too. Don't just fall back on templates, generic characters, and auto-generated designs. If you aren't sure if your animation art sucks, ask someone. Ask someone who hates you. If they say it's good, you're on the right track. Or they're lying so you'll embarrass yourself.
3. Bad animation.
A certain 2D animation program I won't name drives me absolutely crazy in that its boning system encourages people to create these stiff, wooden animations of square-shouldered mannequins standing in one place while their arms jerk around and their heads bounce like bobbleheads. Worse still is cruddy hand-drawn animation that barely bothers with 4-5 frames per second, though for the most part the realm of crappy, lazy animation seems to belong to Flash.
4. Tasteless humor and obscure jokes.
Off-color humor has its audience. Ren and Stimpy proved that; so did Beavis and Butthead, Family Guy, and numerous other animations that tread the edge of good taste. But the key is that they tread the edge, sometimes teetering so dangerously it's thrilling. (Okay, Beavis and Butthead and Ren and Stimpy went over that edge rather often.) But they did so cleverly, and even when being tasteless they did it with enough wit to make it excusable. Just being tasteless for the sake of being tasteless without any intelligence or humor will turn people off. So will obscure jokes that only make sense if you've read page 72, line 3 of a treatise by Carl Jung and remembered the exact wording. Humor in animations needs to be accessible. Not necessarily to everyone; again, Ren and Stimpy and Beavis and Butthead are prime examples of that. But it does need to be accessible to its target audience, and show an understanding of their expectations.
5. Bad music and voice acting.
I love watching badly-dubbed foreign films; they're hilarious. Not so hilarious are animations with lackluster, unprofessional voice actors who can't stop mushmouthing over the script, and godawful music that doesn't fit the scene and that often drowns out the (horribly acted) dialogue. Another hot mess is when you can't find good sound effects and trying to create your own by plunging your hand into a jar of mayonnaise. If you can't afford to hire professionals for voicing, scoring, and sound effects, check out what royalty-free resources you can for audio, and try to audition a few friends with talent. There's always Craigslist; it's full of aspiring hopefuls willing to work for free in exchange for a credit. There's axe murderers, too, but you know how to screen for those, right?
6. Bad timing, pacing, and overall storytelling.
Animations that drag on with nothing really happening won't captivate anyone's interest. Bad timing isn't just about what doesn't happen, though; it's also about not timing what does happen correctly. Like trying to show a punch in slow motion by just slowing down the frames, instead of actually animating the precise timing required to give it the right impact. Or not bothering to time anything and just using the exact number of sequential frames for every action, so that a swift sword slash is detailed in the same number of frames as a walk cycle. No. Just no.
7. Extremes included just for shock value.
We all saw the Saw films. We don't want to see them recreated in your animations. That goes for your demo reel, too, by the way. Just don't. If you're just putting it in there to horrify people, you'll probably know the exact second when they'll stop watching, but won't understand just why they didn't understand the stunning conclusion of your genius masterwork.
