Even more important than "Elevator Tag Lines" (what's an elevator?), are ideas for reaching TeXers no other site has yet reached.
I'm probably biased here, but I think that one great segment of TeX users for us to get would be those who use TeX as part of their job, but aren't so keen on asking questions on forums or mailing lists. I think that the easy entry and ability to focus just on the interesting questions are specific benefits that will appeal to such users.
I have a suspicion that there are a large number of academics in that category. So telling them about tex.SX would be a great way to promote the site.
However, as that community is not so au fait with the internet (by definition!), simply posting adverts on forums and blogs and mailing lists is not the best way to reach them. I thought that having a poster that can be shown at conferences would be a good way to do this, but I'm open to other ideas of course!
If a poster, then obviously it has to be prepared with TeX and in content it should be neutral - but the neutrality should be by mentioning stuff rather than avoiding mentioning stuff. Stuffiness should also be avoiding, lots of eye-catching graphics and phrases, maybe some questions taken from the site, and some answers as well.
So this is a Call for Suggestions, as well as a Is This a Good Idea?
: There's a hilarious spoof episode of the radio soap "The Archers" that ran during an British Election season wherein, due to the BBC's strict neutrality policy, if a political party was mentioned by one of the characters then every single other party had to be mentioned as well.