First thing: Watch this video from Luis von Ahn http://www.youtube.com/watch?v=cQl6jUjFjp4
He is the creator of reCAPTCHA and working on a translation solution which answers your question (regarding how to get people to eagerly do work for your community) pretty eloquently.
Regarding your specific user engagement problem... I would venture to say that the problem you're solving here is not clear. You describe what your site can DO! But not what your site DOES for me. In other words, you're solving problems. Not providing solutions. This blog post from Dave McClure is pretty relevant here, too: http://500hats.typepad.com/500blogs/2009/08/your-solution-is...
Despite the aggressive tone of the article, I think there's a good insight on how you should be "pitching" on your website's landing page.