Well, I have sworn off posting on this board, but I will break my silence this one time since you are asking an honest question.
IMO, you should not be asking the question "why should I not vote for Kerry?" I have never been a supporter of voting against a candidate, but voting for a specific person. Whenever I talk with candidates, I say to them, "Don't tell me why I should vote against your opponent, but tell me why I should vote for you." If they cannot do that, then I cannot support that candidate. They are not telling me what they will do for me, just ripping their opponent. This is a good indicator that they really have no convictions or plans for the future, just "vote for me because I am not the other guy."
Paco makes a good point. You should prioritize what is important to you. Then see how the candidate feels about those issues. The main thing you have to do, is get past the campaign rhetoric. It is extremely difficult to find non-biased sources, but for Senator Kerry, just go to http://www.senate.gov to check on his voting record and for his speeches in front of the Senate. His speeches will give you a better indication of his beliefs, since voting records are not always a good indicator of the candidates position (he could be voting against something in the bill, but may be in favor of other parts) For Bush, look at the legislation he has proposed. Do not look at the final bills, but the proposed legislation. The final legislation may be a compromise, and not a true indication of his positions.
I can give you many reasons why Kerry and Bush should not get your vote, but they may not be reasons that are important to you.
I hope that this helps.