The current title (tooltip) text on the upvote button says
This answer is helpful.
The current title text on the downvote button says
This answer is not helpful.
Since "helpful" is such a subjective word (and kinda bland), these messages do little to discourage popularity-based upvoting, and voting based on programmer glurge1. Additionally, both titles presume that the "answer" is an answer to the question—when in reality it's just text in the Answer box that might or might not, in fact, be a real answer. This dilutes the meaning of the word "answer" on the site.
So, given that a core value of StackOverflow (et al) is to generate quality answers to questions...
I'd like to suggest that a better title text for the upvote button is
This answers the question, and is accurate or helpful.
and a better title text for the downvote button is
This does not answer the question, is incorrect, or is not helpful.
The subjective "helpful" bit is there at the end, as a catch-all, but first the user is encouraged towards more productive voting practices.
Would this be a (ahem) helpful improvement to the site(s)?
1 – For instance: "Gosh, I just luvs jQuery, so I'm upvoting the glib 'use jQuery' answer. jQuery is kewl. What was the question again?"