Dropping a title immediately? For me it's usually bad art. If the art is very amateur, and there's a lack of care with things like text/speech balloons.
With scroll comics, I'll drop it if the amount of scrolling is too high. If my thumb/mouse wheel finger gets tired because all your panels are fade to black with single words or otherwise so low information that it's not worth stopping, I'll quit right there.
I'll also usually drop a comic if there's a Q&A or a large ad (like larger than the comic) before the first chapter/story arc is complete. Dude, you have 1 'chapter' with 30 views and 2 sub4subs, let's learn more about your world before we jingle the beggar's cup and asking for the audience to have worthwhile questions about your world/process/narrative.
So, professionalism, respect, attention to detail, quality. That's the main thing.