Trump can't win a general election.
Actually, I wouldn't underestimate Trump's chances of winning the general election if he's nominated, and Hillary and the Dems would be making a big mistake if they did. His chances of winning the nomination were already underestimated enough as it was, in spite of all the crazy things he's said and done already.
There might be enough independents who are nodding their heads at all the crazy things Trump is saying. There might be enough "anybody but Hillary/Bernie" voters who will hold their noses and vote for him. There might be enough people who simply think he's the lesser of two evils in the general election or that his business experience trumps (sorry) some of the extreme rhetoric he's spewed. The young people who showed up to vote for Obama probably aren't going to do the same for Hillary, especially if those young voters wind up thinking Bernie got a raw deal in the nominating process. He might have an outside chance of winning 20-30% of the African American vote since he's not a traditional Republican, and if he does that, he's going to have a big leg up on Hillary.
Nobody's really been able to figure out Trump's appeal yet, and we might find out that it extends a lot further than just people showing up for Republican primaries for the first time to vote for him.