Not enough character development, too much mythology, and hanging onto characters that absolutely needed and deserved to be killed off. What made the first season so amazing was the connection to the everyday. We knew these characters, but we didn't. It was fresh and not bogged down in the entanglements of sticking to mythos. Keeping everything vague and unexplained (while still having a clear protagonist and idea as to what the 'final conflict' would be) was what made it so riveting. Later seasons introduced throwaway secondary characters with such great potential that get killed off before the season ends (Knox, Adam, Claude, etc). A season of Heroes should end with about half of its MAIN cast dying. Nikki, Claire, the entire Petrelli family. F
ck em all, start over. Keep it fresh. But they instead made the entire storyline overwrought with complicated mythos (combined with too large a cast to prevent any significant, non-cliche character growth) that is what kills most major Sci-Fi series and comics later on.