Because certain core philosophies are enough to push you one way or the other.
I can be pretty sure McCain will be causing damage to things that I care about.
His stated goals are definetaly in opposition to mine, at least in a couple of areas I really want addressed.
I can't be sure what Obama will do, but I generally agree with his stated philosophy, and he's smart enough that I don't worry about him doing anything exceptionally stupid.
This is based on the facts I am aware of at a very high level.
Their day to day actions show plenty.
Maybe it's all bullshit, since Obama has spent a life of being very careful what he says. He can argue both side of almost every argument, and make people think he agrees with them, simply because he is capable of expressing each side of the argument in terms the believers agree with. This might be slimy politician to you. To me, it is an example of someone who tries to think through all sides of an argument before coming to a decision.