I'm a bit late to this, and despite that I haven't come close to reading everything that people have written so I'm not sure whether people have covered this or not, but that disclaimer aside:
The interpretation of the Virginia governor's race, with Terry McAuliffe defeating Ken Cuccinelli by (based on results so far) 2.5 percentage points, appears to be dominated by questions about why it's so close -- rather than questions about why McAuliffe won.
That's because of two things. One is because polls had McAuliffe with a bigger lead; it was estimated at 7.2% by HuffPollster, which is pretty much the gold standard for these things. So there's certainly something to be explained about the difference between the polling and the actual results.
The other is the order in which the votes were counted. Republican precincts reported early -- which is the normal pattern in Virginia, as veteran election-watchers were saying all last night -- and so Cuccinelli actually was leading until very late in the counting. It's impossible to prove exactly how this played out, but I'm fairly confident that if the votes had been counted in the opposite way, with McAuliffe getting out to a big lead, settling down for most of the night into the 5-7% range, and then Cuccinelli catching up quite a bit after the networks had called the race and the candidates had given their speeches, that the election would have been perceived as a much more solid win.
Getting back to the first step here: the legitimate question here about the difference between the polls and the results is not the same question as the question of who did well and why. Which is why I agree with Greg Sargent and Ezra Klein that it's a bit bizarre to call the election a win for ACA foes.
As far as the polling shift, here's my two cents that I'd begin with. Compared with the HuffPollster estimate, the actual results (so far -- remember that they sometimes shift for several days after an election) are the following changes:
McAuliffe + 2.7%
Cuccinelli + 7.4%
Sarvis - 2.5%
If I'm reading the exit polls correctly, most voters who supported the Libertarian Party candidate would have supported McAuliffe in a two-candidate race. I haven't looked into the details of the pre-election polls, but at least one strong possibility is that conservative libertarians (or perhaps conservatives who just didn't like the Republican candidate) came home, while liberals did not. That certainly could have something to do with the themes candidates campaigned on in the final days, but I'd suggest that Ron Paul's high-profile appearance for Cuccinelli is a more likely suspect. And then I'd also guess, and this is a bit more speculative, that Democrats were more likely to trust the polling (and therefore assume that it was safe to vote against a candidate they preferred to win) than Republicans (who, believing it was a close race, came home even if they weren't thrilled about it).
One more thing. While I was checking that one in the exit polls, I noticed something that I did not expect at all: outgoing Republican governor Bob McDonnell had a pretty solid 52% approval rating among exit polled voters. If our interest is in who won and why -- and not in the real but less interesting question about the polling -- then that suggests that Virginia was in fact probably ready to elect a Republican, and that the net effect of the candidates and campaigns probably favored McAuliffe, not Cuccinelli. Which is certainly long way from nailing down the effect of any specific issue, of course, but does suggest that a focus on "why was it so close" really is a backwards way of trying to understand this election.