I'm not an American, and I didn't pay too much attention to American politics until I turned 18 (so I've only really paid attention to the last two elections) but I really think there are several things that will sway how this election will turn out.
First off, the ecconomy will be interesting to watch because it is unhealthy and the fed is dangerously injecting money in an attempt to prevent a collapse; typically Republicans have been trusted more then Democrats in poor ecconomic times, and right now the Democrats seem to be planning a "new deal 2" which most ecconomists think is a bad idea.
Secondly, there is the question of who wins in each of the parties and who they choose to run with them; in order to create a completely balanced platform, the potential "vice president" is critical. There are certain pairings for the Democrats and Repubilcans which could be good whilst others will be awful.
The final unknown is the skeletons in all the candidates' closets; you don't get to be in the position any of the candidates are in without having a (somewhat) dark past. Unlike a lower profile election (senate, congress, or mayor) there are so many people digging through their past that their skeletons will be found and will be brought into the open with a ton of spin from the press. As Dan Rather can tell you, there is also a risk that people will make up stuff about the candidates, if they're successful it will hurt the candidate while if they are unsuccessful it may hurt the other candidate.