If you ask me Heroes problems began appearing during the last two episodes of season one and have just gotten worse ever since.
There is huge potential in this show but I just feel its poorly planned and poorly written. Turing Sylar into a good guy isn't a bad idea but the way they have gone about it (lets just make him a Petrelli) is so poorly done. I'm pretty sure you can find fanfics out there which handle the situation better.
I swear to god if I have to hear Noah grab Claire one more time and say 'EVERYTHING I DO I DO FOR MY FAMILY!' I will give up on this show. Or have to watch the Petrelli brothers hate their mum. Or see Nathan start the season in a hospital bed.
This show repeats itself too much. I mean in EVERY season so far we've seen Hiro and Peter go to the future and see its destruction. Cool at first in season one - boring in two and three.
In season one the characters seemed fresh and followed logical paths. Since then they have either returned to status quoe (Claire, HRG, Angela), followed illogical paths (Mohinder a bad guy, Sylar a Petrelli) or simply been killed/written off because the writers know they have messed up so bad the character cant be saved (Nikki, Maya and the black chick from season two)