The Holes in the Polls
There are two wild cards in this election that the pollsters are unable to deal with.
The Bradley Factor
Named for former L A Mayor Tom Bradley who ran for Governor of California in 1982. Polls showed him way ahead but he lost. The general interpretation is that people who were polled didn't want to sound racist and so told pollsters that they were voting for Bradley, but that on election day they voted against him.
The Likely Voter Factor
Polls reflecting the views of "likely voters" are usually considered more accurate than polls reflecting the views of registered voters. A "likely voter" is determined by the polling organization based on a set of variables such as voting history (e.g. "Did you vote in the last Pressidential Election?"), race, age, education, etc (older, white, college graduates are seen as more likely to go to the polls than younger, black, non-college-grads).
However, in this race, it is quite possible that there will an upsurge in young voters and black voters. If so, the "likely voter" numbers will be off the mark as a predictor.
What does all this mean? We know even less than we think we know about the outcome of the election.