Obviously the topline numbers in the "Big 3" races are key.  But here's a few other things I'll be keeping an eye on:

(1) Fairfax County, VA.  The big story in Virginia over the past decade has been Northern Virginia's slow movement toward the Democrats, which accelerated in the latter part of the decade.  Some of this movement is undoubtedly "real," as the county becomes more diverse and more "city-like."  But I've suspected that a good portion of it was more of a reaction toward George W. Bush's Presidency than real affection for Democrats.  This race is only one datapoint, but it may well be an important datapoint pointing toward slower movement toward Democrats.

(2) CA-10.  This race has received surprisingly little scrutiny.  The Lieutenant Governor of California, John Garamendi, should win this D+11 district walking away.  But the one SUSA poll conducted showed him leading a relatively unknown Republican 50%-40%, and the four major Democratic candidates totalled only 56% of the vote in the open primary election.   If the national mood really is swinging heavily against Democrats, then this oddly-shaped suburban district is exactly the type of district that I would expect Republicans to make gains in (and by gains I don't necessarily mean an outright win).  If Garamendi falls below, say, 54%, it isn't a great sign for Democrats.

(3) New Jersey Assembly races.  The bizarre 3-way race at the top of the ballot makes it hard to draw any firm conclusions about any outcome of the race.  Keep an eye on the downballot races here to get a sense of where the state is really pointing.

(4) Chapel Hill Mayor's race.  Chapel Hill is the Berkeley of North Carolina, has always been the Berkeley of North Carolina, and it hasn't elected a Republican to its town council in over a century.  It won't do that this time either, but there is an Independent candidate who, by Chapel Hill standards, is an outright conservative who is in a dogfight for the mayor's race.  Matt Czajkowski is running on issues like enforcing the town's loitering and panhandling ordinance, lowering property taxes, and enabling development.  Czajkowski voted for Obama, so I don't want to overstate this race as a sign of a Republican upswing in Chapel Hill.  But the fact that he's competitive in this race is more of a signal of just how sour the mood is in the country right now, and how focused people are on economic issues this cycle.