A no mercy list for beginner's editing mistakes
Beginners just aren't ruthless enough. Get rid of all those shots that are 'kind of good' until you're left with just 'good' ones. Remember: if in doubt, take it out. You can always bring it back if it doesn't feel right without it.
If your test audience doesn't understand part of your film, change it or cut it out. Otherwise, you have to be there at every screening to explain to the viewers: 'Well, that bit was actually meant to do something else, but I couldn't quite get it to work!'.
Silence can sometimes be just as powerful. It's all about variation
Most effects look quite cool the first time you try them. After that, they just look like cheap effects. Nothing screams 'beginner' as loudly as unmotivated effects & wipes.
Out-takes can certainly be funny, but having a section of out-takes longer than the film itself will be funny only to you...
No good film is too long and no bad film is short enough - Roger Ebert