I think this is pretty different. The big reason that people did not expect Trump to be the favorite for the nomination right now is that they thought the party establishment could exert a lot more control over the process, and in 2012 we saw them pretty effectively eventually force their guy to the top even though some weird insurgents did very well in the polls at one point or another - Newt Gingrich even won South Carolina. The party is a bit weaker this time around, but I think what a lot of people missed early on is that now there's room for two insurgents simultaneously. Cruz is attracting conservative activist types while Trump attracts disaffected racists. It's been very hard for establishment candidates to get momentum, but without Cruz in the race this might have gone very differently. And even now there are two establishment candidates in the race who are stealing votes from each other.
The reason people don't think Trump can win the general election is just that they don't think that there are enough racists. Just as with the fractured primary, this goes out the window if, say, Sanders mounts a third party run after losing the nomination. But the thinking is that some otherwise reliable Republican voters would not vote for Trump and that women and minorities will be especially enthusiastic about stopping Trump. Actual swing voters are less important nowadays but they too probably aren't huge on Trump - these are people who probably voted for Obama.
These are pretty different kinds of theories, and I don't really see that "Trump did the unexpected once" makes it that much more likely that he'll do it again.