I have to agree with Ozymandias
that such a Slashdot-style voting system wouldn't benefit the site in any noticable way. My reasoning is different though; with the way the system currently stands, a post's reputation has next to no effect upon who reads it or how someone regards it, since you can't see the reputation until after you've voted for it yourself. The exceptions, naturally, are the Best Nodes
and Worst Nodes
pages, which let you jump the gun. Newbies especially, the folks without many votes and those most likely to learn from others' evaluation of a post's worth, don't ever get to see the reputation of the vast majority of nodes. Without seeing the reputation for all answers to a question, an unexperienced user would be unable to determine which was the best. Categorizing votes wouldn't help newbies at all, IMO, since it would just be a complication of the current system which doesn't help newbies at all.
Of course, this is all by design, for better or for worse. From Voting/Experience System, "Now you're probably wondering what are levels and experience good for? Well right now it's just about status, but soon higher level users will be able to approve, cool, and delete writeups." The later powers haven't yet come to the masses (at least, not Abbots and below), so voting is valuable only for rating the poster, not the post.
I've advocated in the past allowing a "bye" vote, which would allow you to forfeit your voting chance on a node in exchange for seeing its reputation. This would be useful for the (many, in my experience) times when you don't like a node enough to vote it up but certainly don't want to vote it down. I'd often like to know what other Monks think of such middle-of-the-pack nodes, despite a wavering opinion myself. Newbies especially could make good use of a such an extension. A person allowed to view the reputations for nodes about unfamiliar topics would inherently familiarize himself more with the more correct approaches. I know there are many holes in my Perl knowledge which would benefit from such a system.
Back to the proposal at hand then, until a user can make a fair comparison of the reputations of different replies, requiring categorization of a vote would further clutter the system without any payoff.