I think it's an interesting idea, whether or not it even works. It would be a unique expiriment. Advertisers couldn't complain because, after all, why advertise somewhere that people not only dislike advertising and ignore it, but dislike YOUR advertising specifically? Spend your money elsewhere.
However, another way of voting is already in place. Click-throughs. If nobody clicks on an advertisement, chances are it's not going to hang around as much as if dozens click on it. Seems obvious?
Finally, here are some advertisements I do not mind seeing. In fact, I enjoy them:
Programming and other technical books. Even the occasional social-engineering type of book. Not just anyone who has a book to sell, but something that one or more (educated) people have read and perhaps even reviewed and would reccomend. Advertisements for the K&R book? Good. Advertising for O'Reilly's How To Use Windows 95... bad.
Advertise cool geek junk. ThinkGeek is good at this. Nerf toys, legos, books, MP3 players, coffee stuff...
In other words, if possible, don't advertise the obvious (Redhat... for instance). Advertise stuff that will make even us go "whoa! fucking sweet!". Slashdot is better at that than your average website, but I think K5 could be even better.
If you do set some sort of voting mechanism up, make the scores viewable by everyone, including WHO voted which way for each item -- that way nobody can claim that K5 is pulling a fast one on us and telling us "no, this Victoria's Secret ad really was voted up!" (Hm. Actually, that doesn't sound too unlikely! heh. ).
Also, maybe you could have a page where you put advertisements that are pending -- or even currently running so those who are interested can browse and vote all on a single page.
Basically, I'm just saying to take a pro-active role when at all possible and choose to advertise things you would consider worth it -- not just whatever pays you money (if you can).
One last comment -- I think more "geeky" stuff would go over better than "corporate" stuff. More of us are probably interested in cool lego toys, good books, interesting ways of consuming caffeine and fun ways to hurt each other without causing permanent damage than we are in buying a $2500 rack-mount server for our business.
I just read K5 for the articles.