Well, being a Barr supporter, I'm not going to tell you to vote for McCain, but I will give you some reasons to not support Obama.
First, Iraq. When listening to Obama, you really have to listen to his exact wording. He usually says he's going to take the "combat troops" out of Iraq. This is important, because his obamablueprintforchange.pdf says he's going to ensure the safety of Iraqis in and around Iraq. Of course, that's going to require troops...but they'll be called "peacekeeping troops" rather than "combat troops." So we'll still be in Iraq. He's not going to take the troops out.
He also wants to get the US involved in the Darfur conflict.
He wants a "civilian national security force", which will be "just as powerful, just as strong, just as well-funded" as the military
http://www.americanthinker.com/2008/07/obamas_civilian_national_secur.html
He uses "truth squads" to stifle free speech
http://www.youtube.com/watch?v=xCp0zlsVxOQ
He supports the PATRIOT Act.
He supported the telecom immunity bill, which also gave law enforcement even more time to listen before requiring a warrant.
He's a gun grabber
http://johnrlott.tripod.com/op-eds/FoxNewsEvalNRAAds092408.html
http://www.washingtontimes.com/news/2008/aug/29/obamas-gun-ban-rhetoric/
He'll be bad for the economy
http://www.newsbusters.org/blogs/noel-sheppard/2008/10/09/74-ceos-believe-obama-would-be-disastrous-nation
http://www.cato.org/pub_display.php?pub_id=9746
Clearly, a better question would be - why should anyone vote for Obama?