In addition to the Senate races that generate most headlines (such as Arkansas, Louisiana, and North Carolina), James Hohmann draws attention to five races that could get very interesting. Hohmann suggests that analysts should keep an eye on Virginia, Minnesota, Oregon, New Jersey, and Kansas. Four of those five (excepting Kansas) would be Republican pick-ups.
UPDATE: Hohmann's piece has proven somewhat prophetic, as Kansas has now indeed become more interesting. The official Democratic candidate has dropped out, throwing his support to former Democrat (and current independent) Greg Orman, a businessman who is challenging incumbent Republican Pat Roberts.