How was it a surprise? With huge crowd sizes at his rallies (that NYT didn't show, but RSBN for example did), strong populist message that worked, and the awful candidate the other side put up (while sadly destroying Bernie), I was not surprised.
Certainly an underdog, but with a reasonable chance of success.
Also, it was clear on election night that neither Hillary nor Trump had prepared a speech for a Trump win. (Watch Trump's victory speech from election night. Hillary had to awkwardly postpone hers till morning because she didn't book the venue long enough to handle a close race.)
I suspected 538's projections were spot on because of math and stuff, but many (most?) people did not.
Until they find a way to poll a significant slice of the population truly representative of the electorate, I'm surprised people can take them seriously. Since we're on HN, there is a startup idea here somewhere.
(Edited to remove an electoral hypothesis, so that we can discuss polls and not politics.)
As easy as it is to critique Obama's policies on privacy & foreign policy, he was assured and rational, which people like in a leader. It's entirely reasonable that there's such a wide gap in approval polls. There's really no reason to significantly doubt them.
Regardless of whether you agree on the above, we're about to see the GOP have its way and push a slew of policies that they favor. Many will actually be favorable for entrepreneurs and startups, but my guess is that overall they will turn out to be quite unpopular. I predict the house to flip back in 2018 and for Trump to be crushed in 2020 (if he makes it that far without being impeached.) We'll see.
Plenty of things that are "common sense" don't pan out once you throw in gerrymandering and voter turnout.