5
I've found that a lot of people don't like various SJ pieces due to filler, being drawn out, weak, generic, or annoying characters, predictability, and length. Those are mostly the reasons I don't do Naruto and Bleach anymore (not One Piece, though; Oda does very well when it comes to developing likable characters and touching stories filled with unpredictability), as well as a few others (of course, not all of them fall prey to this; a lot of them don't).
I'd never dislike something simply because it's long, though I prefer them to be shorter than they usually are. My ideal length for a Shonen is Yu Yu Hakusho length (well, its 24, but I have zero problem watching an anime/reading a manga as good as YYH for as long as it ran), and I really wish Bleach/Naruto/One Piece/others would end- not because they're bad, just because I feel an ending could only benefit them...unless it were rushed.