There are many common problems with lots of fanfics, a lot of which have become tropes, such as the Mary Sue (which I still refuse to acknowledge), flat storylines, et cetera.
Personally, I feel that a single problem trumps any other major problem by a mile. This problem is a problem that I term "skipping".
In a nutshell, this problem is a lack of detailed description of events, making a story seem more like a summary. Almost any fic that I determined to be "boring" or "badly written" was a fic with this problem, or tried to correct this problem by inserting in a lot of unnecessary details, which simply makes the story seem like it's jerking from one place to the next while moving at the same speed as a slow but smoothly flowing fic should. (If you've ever tried speeding up a video 3x and then slowing the sped-up video back down, you'll know what I mean. The story seems choppy and turbulent.) Hell, my own fics had this problem at first.
But I realize that other people may have differing opinions. What do you feel is the biggest problem in fanfics, if at all there is one?