Clean and simple, yet powerful design. It should fall in line with twitter/facebook logos - unique typeface and font, simple, quiet, powerful.
If you create a picture/logo of some kind, it should be something INCREDIBLY simple and not very rich ala twitter's bird logo. We aren't super intent on having a logo but won't rule out if someone gets it right.
We don't want something that's overly designed or very aesthetically rich. Again, some of the best web service logos are simple and powerful (Facebook, Twitter, Digg, Microsoft, etc).
Demographic is web 2.0-y type users but still want to convey seriousness in the brand and not that it's not some unprofessional project... but we still want to make it fun looking. :)
It would be helpful if I could see the logo in a couple of different colors/background combinations. i.e. When you're logged in, Twitter has the white with a slight gradient on black with the blue bird logo in your profile page. When you're not logged in, Twitter has a slightly dropshadowed flat logo with a white/gray gradient. Above all, it should be subtly styled.
It would be nice if you created a simple logo/icon. This would be something that could be embedded in a site or act as a favicon.
A unique font is the most crucial element of this logo!! All great web services have immediately identifiable, unique fonts. This will make or break every design submitted!