Over at Pharyngula, there's an ongoing discussion about Barack Obama vs. John McCain (This is how Obama could make me happy to vote for him).
I see a huge, if not the, important issue in this election being the balance of the SCOTUS. The new President is going to appoint at least one liberal judge almost immediately (Stevens, I think, is hanging on until there's a Democrat in the Whitehouse), and maybe two (+Ginsburg?) in his first term. If McCain gets in, I see one or two more Nino Scalias being appointed, tipping the balance firmly to the right for a long time (Scalia won't be the same age as Stevens is now for almost 20 years). The consequences could be church/state separation being compromised, Roe vs. Wade overturned, gay marriage being torpedoed, and even successful appeals to recent judgements disallowing educational crapola in schools. It could be devastating.
As an international spectator, I selfishly want Obama because I think he'll be able to use his mixed-race, mixed-heritage, international background, foreign policy education, and civil rights record to help build peace and rehabilitate America's overseas reputation in a way that another old militarist Republican WASP, up to his oxters in political debt to the hawks and the religious right, simply couldn't do. From an international perspective, Obama is damn-near perfect: realistically, the international community couldn't ask for a better prospective President.
But it strikes me that, from an American and, ultimately, even an international perspective, the SCOTUS issue could be even bigger in the long term: I really don't want American core values (1st Amendment, particularly) and education compromised for a generation.