Let me briefly explain the difference:

With mturk, you have work you want to get done, and people go look through the HITs for things that fit their abilities.

With fiverr, you have money you want to spend, and you go looking through things people imagine someone might want to pay them to do.

I've used mechanical turk for annotating a training set for automated classification. I cannot imagine what I would use this service for. Claiming this is what mturk should have been assumes that mturk sucks because all the tasks people want to get done are boring.