One blog posts looks at how well different profile attributes predict whether two people will match. Not surprisingly, the zodiac signs of any two people have no effect on their actual personalities, and thus on how well they would get along:
Race has a slightly greater influence (of a few percentage points either way), presumably because of uneven distribution of cultural backgrounds, but it is still fairly small. (Keep in mind that the match scores are computed from how users answer others' questions, and not from explicitly asking questions like "would you date a Virgo/Polynesian/Buddhist".) Religion, however, turns out to be a lot more telling:
According to this, atheists, agnostics, Jews and Buddhists seem to get along just swell (in fact, Buddhists appear to be slightly more compatible with the nonbelievers than with other Buddhists), whereas the Christians, Hindus and Muslims tend to be somewhat more contentious, not only not getting along with other religions as well but also with each other. Additionally, the more seriously one takes religion, it seems, the less likely one is to get along with others.
Looking again at the issue of race, while race doesn't seem to affect actual compatibility scores, it does affect how likely people are to get responses:
Love may be blind, but it also seems that it, or at least attraction, is deeply racist.
On a lighter note, OKCupid have crunched the word frequencies of successful and unsuccessful opening messages and discovered what to write if you want a reply. Netspeak and "hip" misspellings ('u', 'luv', 'wat') and physical compliments are out, whereas mentions of specific interests are helpful. Unsurprisingly, mentioning religion is generally a bad idea as well.