Heroes

Peter should not have such a powerful ability. He can hold more than one power, but he cannot just stand by someone to gain their ability. I can see why they made him so powerful the first time around; it sounds really cool, but it actually is too much. Sylar stays exactly the same.
At the end of season one, don't have Sylar get stabbed in the chest. I was very annoyed to see he had lived in season 2, only because people that die need to stay dead.
Season 2, don't have Suresh get powers. He becomes so unscientist. Also, have some other random voyage for Sylar to be on than getting picked up by Maya and brother. That train of thought just wasn't beneficial. Maybe have Sylar get picked up by a terrorist group or the mob; THAT could be cool. The story of Adam whathisface and Hiro finding him in time was really good, along with Adam's association with "The company."
Volume 3 (the first half of season 3) needs to be completely rewritten, bottom up. Mr. Petrelli being alive ruined it; villains can be brought from anything, but like with Sylar, don't bring back the dead. It gets so annoying.
The second half of season three was good until everyone despaired about Nathan dying, particularly in season four when Hiro was dying and Claire offered her blood to help. She can bring back dead people, she already brought back HRG. They could make a cute little joke about her bringing back both fathers.
Season 4 actually was also very good, but it lacked action.
Lastly, and overarching all of this, is keeping abilities consistent, as slightly referenced to with Claire. There are too many instances where a hero can do something in an episode, but cannot replicate that same action in a later episode, and vice versa. It leaves all of us yelling at the screen.