A thing we have to remember about tv shows.
It is much harder to get a new series going that get a hit series renewed. I think what may have happened with Heros (Lost and Prison Break can be included in this) is that the creators had to work really hard to get the series made in the first place. The story had to be perfect and the show actually had to be good.
Maybe the writers spent all their energy and ideas on the first season not even knowing if it would get made. Then suddenly they're a hit and get picked up for 20 more episodes the next year and can't think of anything good for the show.
I absolutely loved Prison Break season one but I couldn't even stomach season two. Lost seems like a show I would really like and I was going to rent season one and two on dvd to get caught up, but then I realized, what's the point. They don't get off the island. No matter how compelling the show is at the end of the season, they're still there. Stuck. Just like Gilligan.