Duolingo -- the next chapter in human computation | Luis von Ahn | TEDxCMU 2011

First thing: Watch this video from Luis von Ahn this video 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: link 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.

I am currently using Duolingo to learn Spanish and I find it quite amazing. It offers a great interface and I love the gaming elements. It's by far one of the best web applications I have used in a long time. So kudos to Luis von Ahn and the team! I recommend watching his TedX talk (the mind boggling thing from the presentation is: English Wikipedia could be translated in 80 hours if 1 million people used Duolingo!) this video