Lots of $1 spam.
I like the idea of the solution being presented. This way you do not work only for money, but money do come as a bonus
One difference is that users are encouraged to place bounties for existing issues on open source projects issues - the main goal on FS is to kickstart Free Software projects - but you can place a bounty for an "unlinked" task much like Bountify, like this one --> http://www.freedomsponsors.org/core/issue/13/a-complete-jee-...
Another big difference is that there's no money escrow. You pay after the task is done, if you're happy with the result.