What’s the “elevator pitch” for our site? See: http://blog.stackoverflow.com/2010/07/the-7-essential-meta-questions-of-every-beta/ #4.
Exercise your right to homebrew with confidence.
It's a little take on prohibition and being able to be confident in your brewing due to the help you receive from other brewers.
This might be more of a tagline or motto than a pitch but feel free to expand on it.
I think the main selling point should be something like "getting the best brewing answer quickly".
The downside over existing forums is probably going to be a lack of community that can come from a more community oriented site; compare with the Brewing Network 'army', for example.
Key points for the elevator pitch:
*Speed - If an answer exists already, tagging & search help you locate it quickly. If you ask a new question, tagging & search get potential answerers yo your question fast.
*Quality - Multiple rated answers help ensure the most valid answer rises to the top. In surfing various forums and home brew sites around the web, I have seen A LOT of incorrect information, some of which borders on superstition.
*Civilized - The reputation system tends to prevent people from engaging in trolling, flaming, etc. Voting helps filter out trash and wiki functionality allows gardeners to fix a fundamentally good question that may have been poorly asked. (I am actually not sure how pervasive these problems are on brewing forums, but they seems to be endemic to nearly all forums on the 'net.)
(in progress)*Endorsed - If we solve (http://meta.homebrew.stackexchange.com/questions/14/how-do-we-promote-our-site), this can be another key point. As stated in the "7 Essentials", "cult-classic bloggers and celebrities in their field" bring credibility to the site. This is important since credibility is one of the hardest things to get across in an elevator pitch (in my experience at least.)
More inspiration may come from Joel's (I think fantastic) talk at Google - http://www.youtube.com/watch?v=NWHfY_lvKIQ