It kinda depends. I've liked a lot of different types of ff.
I respect writers who spend some time with editing. When I see stories that are overly long, or the descriptions are repetitious or there are certain technical issues, that can be a turn off as it tells me that the author didn't reread, or didn't reread recently. I am not saying that ff needs to go through 50 rewrites, but when the writer continually confuses "to" and "too"; or only says "he said" throughout the piece instead of sprinkling in "he whispered", "he stated", "he opined", "he confirmed", etc.; or drags out a slow-moving scene to 20 pages when it could have been told in 2 - that tears it for me. I want authors to care about the reader enough to get things like that right or at least make the effort. I recognize that a lot of people are somewhere along the learning continuum, and not everyone is a native English speaker. I get that. But these are things that writers need to learn.
I love it when writers get into a character and can really get the dialogue right. E. g. Spock, Data, T'Pol and Tuvok rarely use contractions or slang. When they do, it's the sign of a problem. An author should know that, and should write their speech accordingly.
I also like effortless exposition. Someone telling me that a character is 6'4", weighing 210 lbs. isn't natural unless that character is in a police lineup. Instead, I want to get that information elsewhere, e. g. a character says he's self-conscious about his weight, or another character looks up because the first one is taller.