Poor grammar can quickly ruin it for me because if it is difficult to read then its not fun either. Im a pretty huge grammar nerd though. Changes in tense and most other small things are easy to ignore or even undetectable if the overall writing is good enough though.
Actual real fanfic question, how strict are you guys when it comes to the literary standards of what youre reading? Like can you excuse poor grammar or wonky formatting for a good story or is it an immediate no or somewhere in the middle. I want to know no wrong answers