Datechguy uses numbers to show why it matters that the party affiliation of poll respondents doesn't match reality.
Short version: pollsters are weighting their polls as if there are more Democrats than Republicans, when the opposite is true. Without this inaccurate weighting Romney would show a significant lead.
This poll shows that have been more Republicans than Democrats for all of 2012:
This chart shows that pollsters consistently weight their polls with more Democrats than Republicans, and thereby skew their results towards Obama and away from Romney.