How about a -1 that affects ranking ONLY to people who enables the feature and follows you?
I mean, if I follow an expert in his area, it makes more sense to me to see that he doesn't endorse spammy-website.com, than to see he endorses "Chrome Beta for Android".
This used to exist - SearchWiki gave you the opportunity to either vote up a search result or to remove it entirely. Both of these actions would affect only your own results - there were vague plans to eventually use the data to improve ranking, but the spam/SEO implications were never solved.
Basically people just used it to kill experts-exchange.com. Usage was tiny otherwise, and the experts-exchange case was fixed when StackOverflow started getting traction and Panda put the final nail into experts-exchange's coffin.
I'm honestly still surprised that YouTube has a dislike button. It must be incredibly important to their algorithms there.
But I'm 95% sure Google+ will never get a dislike button, much like Facebook. Dislike buttons don't belong in social media platforms. They sow disappointment and hurt egos (e.g. karma on reddit).
It would be trivial to check if someone has watched something before they downvoted, and adjust algorithms accordingly (ignore the vote internally and lower that voter's general impact).
I love to dislike things, especially if disliking them would affect their Google ranking (I'm looking at you Quora and your dark patterns!).
Not only I would love the -1 button, but also negative endorsement, and pretty much any Google+ feature, but with a negative twist.
That would really encourage me to use Google+, or should I say Google-.