Nothing (except for badges/karma which are intangible and cannot be translated to $$).

Once you start offering anything tangible to me, I start thinking about how does it translate to my hourly rate and this is where you will fail.

So much has been written about why Amazon has the best reviews ever without ever paying a cent. Why Wikipedia is the greatest encyclopedia ever with last (and only) paid contributor back in 2005. etc etc...

If you start offering people something in exchange for their contribution, you will fail. Just look at Google Knol.