I'd say it went down hill starting with the first year's finale. Which rather nicely exemplifies what was fundamentally wrong with it: great build-up, but they had no idea where to go with it.
It's a major flaw in the "mystery box" story structure so favored by the likes of JJ Abrams. Tease all you want, but sooner or later you're going to have to deliver the good stuff and if the best you can do is a giant magical sink plug of doom (I'd give a more apt example, but I've honestly forgotten how Heroes ended!) then any goodwill earned up to this point will quickly evaporate.
As for bringing the show back....meh. :/
Don't get me wrong, the show *had* plenty of potential, but unless they have a clear, *good* idea of where they're going with it beyond just one season then it'll just crash and burn all over again.