For me, Fanfiction is like food. Some I love, some I hate, and some I'm just "meh" about. Depends on the author and their preferred writing style.
It also depends on the fandom. Some fandoms I'm currently hooked on, so I'll read any and everything written. (The good, bad, and the ugly.)
I'm also a HUGE stickler for spelling, grammar, etc. If the person can compose a sentence in its correct form; that's what makes for good fanfiction for me.
I also enjoy fanfiction if it has humor, emotion, and I can connect and visualize what's going on with the characters. If I can visualize and hear in my head their voices as I read the story then I usually give good praise for the writing.