I selected for but each will take away from a fan production in different ways. Writing is probably the one that stands out the most. A lot of fan productions insist upon writing overlong monologs when shorter sentences would do. Acting is tied closely to writing because even good actors can't make bad writing work perfectly. But, there is times when the actors simply do not sell it. They don't react, barely react, or there is inconsistency where one actor is intense while another is basically a part of the scenery.
Sound is one that I think Maurice is spot on about. As much as I have limited care about video quality, audio quality is a pet peeve of mine. I can hear disparate qualities from tracks, music too loud in the mix, and poor quality in the recording. It drives me nuts and I will struggle because I want to hear so badly. Imagine a distorted video chat and how frustrating that is. Now amplify that frustration because there is no real way to fix it.
Finally is effects. Effects has been something I have seen tank entire productions because the production team wants such big, bombastic effects. And it is such a time sink for something that might not turn out how it was imagined and it doesn't blend in with the shot, so it distracts even further.
One of the big aspects that I learned attempting multiple fan productions is that limits need to be honored. A shorter story is always more welcome than an overlong effort.