Superdelegates have never decided a Democratic nomination.

Really? Because I could have sworn that one of the reasons cited for Obama's win over Clinton in 2008 was his successful courtship of superdelegates. Nevertheless: http://www.npr.org/templates/story/story.php?storyId=89369899

Why do superdelegates get to play such a big role?

They don't — usually. That's because in the past, the battle for the Democratic nomination had been settled fairly early. Michael Dukakis (in 1988) and Bill Clinton (four years later) established themselves as the clear front-runners fairly early in the process. It was even more one-sided in 2000, when Democrats nominated Al Gore, and in 2004, when John Kerry got the nod. The superdelegates really didn't have much of a role to play. But this time it's different. Neither Obama nor Clinton is running away with the nomination. And that puts the supers in a position they have never been in before.

I don't think it unlikely that they'll be in a similar position this time.

Now, New Hampshire is in Bernie's neck of the woods; I'd have written him off completely if he couldn't win the primary there. However, New England doesn't represent all the primaries. The primary to watch is going to be South Carolina: if he can pull off another close poll there like he did in Iowa then I'll be more optimistic of his chances.