I've read loads of long fics that I gave up on, but I can't tell how much is it because they really should have been written differently, and how much they just weren't to my taste.

For me, there's definitely a difference. If I stop because the fic is really not to my taste, I generally don't feel sorry about it, and I most definitely don't continue reading even though it's painful. I do that when the fanfiction is at least partially interesting. Like the stuff between the sex scenes or what little I can spot of the overall plot.

If I really don't like the story anymore I just shrug and go on.