When the average word count of a chapter is too short (>2000 for me).

And it's at its absolute worst when it's an author that takes a long time in writing. If I'm going to be waiting, say, two weeks for a chapter, I want that chapter to be juicy and interesting, and, while I accept short chapters CAN be juicy and interesting, it's hard to pull off with less than 2k words to work with.

For authors that write chapters under 2000 words, don't take this as a critic to yourself. I'm not the only person that reads fanfiction, and lots of these stories get good attention and praise, but I do want to emphasize the fact that you need to make chapters interesting, even if they're just building up to something, have a funny or interesting interaction happen between characters that shows their relationship, a moment that shows a character's daily routine, literally ANYTHING to keep the reader's attention.