Not trying to be too harsh, but since you asked:
What makes your site different than a Stack Exchange site?
How do you plan to entice users to do work, for free?
If you plan to entice them with Karma alone (i.e. no monetary pay-out), how do you guarantee that there will be something they wish to purchase with the Karma they earn? (i.e. How are you going to build to critical mass?)
For that matter, if I'm going to do work for others to earn Karma, and then spend that Karma to hire work from others...why wouldn't I just do work for others to earn money, and then spend that money for a whole range of things beyond just what I can find on your site?
Edit: Just realized I wasn't very constructive there...to expand on my last point: there are downsides to doing work for money (i.e. finding buyers/sellers/clients/contractors, contracts, rate negotiations, tax forms, etc.). So, I'm not saying your idea couldn't work, but as it is presented now, it's just not horribly interesting.