Battle Royale – The first manga I dropped before finishing, I believe. At first I was enjoying the tale of teenagers trapped on an island and forced to kill each other (in the same way you might enjoy a mindless exploitation flick — what new gruesome method of death will they dream up next?), but after reading the highly superior original novel, the over-the-top violence and gratuitous sex seemed even more unnecessary and exaggerated than before. (Keith Giffen’s reworked script, which added a nonsensical reality TV angle to the story and tried too hard to be shocking, certainly didn’t help.)
Made it to: Volume 5, but only because I was still preordering my comics online back then; I’d wanted to drop it after volume 3.
While I have tons of unfinished manga on my shelves, that’s more due to lack of finances on my part than a disinterest in a particular series.
What about you though? When did your favorite series jump the shark?
Update: Johanna offers her thoughts on the matter.