I think Heroes jumped the shark at the start of season three/ end of season two. Sure, season two was rather boring, but it still was the same show with the same feel. Too much, actually. The writers tried to do the exact same speed progression as in season one, going from slow to fast. They were going somewhere, and it got cut short by the writers' strike. When everyone reacted poorly to season 2, they went all whiz-bang over the top, with the story changing focus every other episode and big events that really changed nothing. Someone lost or regained powers every damn episode in volume 3. Volume 4 was better in that regard, but still isn't the same show I signed up for.
(Still, I love Heroes and plan to keep watching. I can see all the problems, I can shout, "Oh, come on!" at the TV all I want, but I still am entertained and still want to see more every week.)