It's an unfortunate quirk of the American political calendar that Virginia is one of only five states that do not select their governor at the same time as either presidential or congressional midterm elections. Political junkies are especially starved for election news in odd-numbered years, raising the danger of over-interpreting the scattered results that do occur. And the fact that one of the two states happening to vote in 2017 is also home to most of the national political media guarantees that attention will disproportionately fall on Virginia, dependably exaggerating the importance of the outcome of its state-level elections. (It's hard to imagine, say, Missouri or Arizona receiving nearly this much notice if it were in the same position.)
The real importance of the Democratic victories in Virginia (and New Jersey, and in scattered locations elsewhere) is not that they tell us much that we don't already know. Instead, they are just a few more data points confirming one of the most dependable rules in American politics: the president's party predictably loses ground in down-ballot races, and the amount of ground lost is correlated with the president's unpopularity.
And Trump is quite unpopular. His job approval rating is the lowest of any president on record during his first year in office, and roughly half of the country not only disapproves but strongly disapproves of his performance. The unexpectedly high Democratic turnout in Virginia tonight suggests that Democratic voters are highly motivated to express their opposition to the president and the ruling GOP when given the opportunity—even in the absence of a personally charismatic Democratic candidate. This is hardly a surprise, but it still demonstrates that the laws of American politics have not been completely rewritten in the Trump era. There were many good reasons to believe that 2018 was shaping up to be a good Democratic year before tonight's results rolled in, and the outcomes in Virginia and elsewhere simply constitute one more good reason.
If there is any actual further importance to the outcome tonight, it comes from the effect that the election returns might have on the calculations of incumbent politicians and potential challengers. If a number of Republicans representing competitive districts begin to mull retirement in response to tomorrow morning's headlines, and if an array of talented Democratic candidates gain enough confidence from these results to throw their hats into the ring for Congress and other offices, than tonight's events will indeed exert an effect on what happens next year. But there is still a long way to go until November 2018, and the nation's political climate cannot be foreseen with precision a full year ahead of time on the basis of a few state and local elections tonight.