Here are three reasons: Poor judgement, abuse of power, and manipulation of the media.  No, I'm not talking about Iraq yet.  I'm talking about his handling of Cindy McCain's prescription drug addiction.  It's not a case of his trying to protect his wife from public scrutiny, that would be understandable.  He was driven by revenge and self-interest.

Supreme Court Appointments - It's not only roe-vs-wade, but issues such as electronic surveillance, gun control, capital punishment, etc... that you must consider.

Middle East - Maybe you are thinking "The Surge" wasn't such a bad idea, but you know what is?  Electing a president who doesn't understand the complexities in Iraq.  We need political progress in Iraq and you cannot expect this from somone who confuses Sunni and Shiites.  You need somone who doesn't look only at how many US soldiers have died recently to judge our success, but considers the impact on our ability to confront and respond to threats elsewhere.  He is a warmonger, at least thats the only reasonable explanation I see for his making false claims about Iran seeking to acquire nuclear weapons when the intelligence community says they did so five years ago.  And, that Iran is training al-Qaida.

Economy - The only thing McCain has is to call Obama a "tax and spend" liberal.  Don't like taxes?  Then don't put them off by adding to our public debt, but remember the interest rates are a killer and someday the bill will come due.  Economists see McCain's proposal going way over budget, so he fits into the recent conservative mold of "spend and spend".  The benefits go to the top, but don't trickle down.
 
In a recent interview in The Wall Street Journal, he was unaware that his Web site endorsed a different plan regarding Social Security than the one he was touting to the interviewer.  It's understandable when you consider something he said back in 2005.

"I know a lot less about economics than I do about military and foreign policy issues. I still need to be educated."  But, “I’ve got Greenspan’s book,” he assured the audience.

Military - If you are a veteran or currently serving, thank you!  If you admire McCain for his service, I suggest sending him a letter telling him so.  But, before you use this as a reason for your vote look at his record.  Not just his stance on the recent GI Bill.  For years he has opposed legislation that veterans and their advocates deem vital.

These are some reasons why not, feel free to add others...