You must either fill a void or carve your own unique niche in business. Entering a market with a lot of similar services means you've got to offer something (more things) different and better than everyone else. You've got one edge, which is that the webmasters get to keep 100% of the revenue. You need more. Think about what it would take to get the webmasters on board (which you've initiated by posting this question) and what you'd need to get the advertising buying (perhaps the lower rates offered by the webmasters who've lowered their rates to reflect the 100% revenue will attract them?). You have two distinct groups/markets you need to know well. Once you've thought about these marketing considerations, I'd also suggest a simplification of the site as well (look at TextLinkAds) that reflects the focus you've (hopefully) clarified. Overall, you may not feel it is worth it, because you've got to try so hard to offer this as a free service. This is a good indicator that you need to either A. Think harder to find and fill a void, or B. Reconsider the 'free' aspect, and perhaps invest more money/resources to develop the idea to it's full potential. Good luck! I hope that at least 25% of what I wrote was helpful!
I think it is a very good idea and am not sure why it hasnt taken off yet. Im sure with a few more webmasters knowing the site, the message would soon get around.
Here's my .02 = Your site doesn't look as appealing as most ad sites out there. Webmasters want easy ads that take a few minutes to set up and so don't like seeing a hugely crowded homepage with a gazillion options. For one, it scared me as soon as I looked at it. Also, for your site to be of any interest what so ever to any interested party - you need a user base already. I guess it's a vicious circle, but why not "pay" people to register or even run a competition or something. As several of the replies have already said - you need to "get the ball rolling" yourself and considering your marketing mix is right, then you should have a nice network there! Rich