Quality of the content aside?
Free hosted galleries/sites is a really big selling point for me. I'll work with a site much more quickly if they give me content I can put up and not worry about. Anything less, and I'll probably forget about your site anyway.
I also like a regular newsletter that has links to said content and zips, with my codes already in place. Makes me FAR more likely to use it, cuz I don't have to: 1) remember your site 2) make it a point to go and get content from your site 3) edit those links so they have my code.
Above all else, improve your tours whenever you can. Make sure they sell... then make 'em sell better. That's, bottom line, most important.
That's pretty much what you'll need to get me to sign up with you.
If I really like your site, galleries/pics of the day are kinda nice too.
Good luck, man.
