I would argue that a couple of the attacks levied against Kerry are excellent reasons to vote for him.
Nuanced. Funny how Kerry's critics are trying to make "nuance" a dirty word. Last time I checked, intelligent decisions about how to run our country and how to handle foreign policy are hardly black and white. I like the fact that Kerry as a senator has paid enough attention to the votes he's making to notice when a completely unacceptable "rider" gets attached to a bill that otherwise sounds like a no-brainer (as many of the shocking "attacks" on Kerry's voting record happen to omit).
Kerry has occasionally changed his mind on policy details after new facts come to light. Perhaps if a team of respected scientists came to Kerry's office with solid and convincing evidence of an environmental harm that could be prevented with a policy change, Kerry might hear what they have to say and change administration policy on that issue. I suspect that if elected, he will end up signing some "conservative" legislation that will piss off the far left for similar reasons: it will make sense. He comes off to me as someone who is open to influence from people outside of his inner circle, as long as they are respectful and argue their case convincingly. Kind of like how almost anything gets done in the Senate when disrespectful partisanship is a one-way ticket to a filibuster. Anybody with a lengthy Senate tenure is going to look "wishy washy" when facts are presented selectively.
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Some additional reasons:
He is running a comparatively clean campaign focused on his own qualifications. Whether or not you're getting that message is more of a function of our dysfunctional media channels than Kerry's effort to provide reasons why Americans should vote for him. Read around in his website and make up your own mind. I support many of his positions, but maybe you feel differently. If you don't support Kerry's positions, don't vote for Kerry (if you feel bad about not voting at all, write in whatever 3rd party schmuck you can find with a view or two you like).
Having the advantage of a "clean slate", Kerry will have a much easier time patching up damaged relations with important U.S. allies, and improving America's image abroad. The Iraq prison abuse scandal is providing way too much fodder for those recruiting the next generations of Anti-US suicide bombers. Whether or not current administration officials are responsible for the culture that allowed those abuses, a different administration will have better chances at the ongoing battle for "hearts and minds", if only because it doesn't bear as much of a perceived stain from these vivid abuse images (I say as much, because I doubt the martyr recruiters will make much of a distinction either way).
For the concerned conservatives out there, odds are Kerry wouldn't be able to turn the U.S. into a socialist utopia (or commie hell depending on your viewpoint) even if he wanted to. Having the legislative branch (likely) controlled by an opposing party does tend to keep government safely restrained. At worst, you're looking at gridlock - but given how evenly divided America seems to be, giving a single party carte blanche for a lengthy amount of time could be viewed as ironically undemocratic (at least from an impartial centrists' view). There are plenty of Americans who "split" their national votes with the intent of providing an additional "check & balance" to our already institutionalized checks and balances. That may not be a persuasive reason to sway your individual vote, but for some it is.
I've tried my best to refrain from directly mentioning why to vote against Bush. I do have to ask however, if Bush was your employee and his annual review was coming up, would you give him a raise based on his Job performance so far? Personally, I would fire him based on his performance, and that's what I intend to do with my vote come November. I'm perfectly comfortable having plenty of reasons both for one party and against the other contributing to my vote, and i don't see anything wrong with that.