Too many Dem incumbents at or below the 50% mark (or upside down in terms of approvals).
True, but I'm sure there are quite a few in the House for whom that is true, as well (there just isn't as much polling).
I agree that the Senate, all things being equal, tends to be more favorable to the GOP than the House, due to the small state bias factor. However, the big problem with taking the Senate is that only 1/3 of it is up for election and the other 2/3 was elected in very favorable years for the Dems (and that 1/3 was elected in a mildly favorable GOP environment), and so it would take a massive wave, and how that massive wave somehow misses the House is the part that I don't really get.
Due to the favorable Senate maps they will be looking at in 2012 and 2014, if I were the GOP I'd rather take the Senate than the House, even though normally the House majority matters more due to the filibuster, because if GOP does take the Senate this year they have an excellent shot at 60 seats by January 2015.
In fact, you could say it is already highly likely that either 2012 or 2014 will be great years for the GOP (2012 if Obama loses, 2014 if he wins), and thus the odds of the Dems having 59 or 60 Senate seats again anytime in the next decade are pretty slim.