From the perspective of an inexperienced Requester on mTurk, there are a bewildering number of informal rules about best practices to keep participants happy. Workers will email you with feedback and/or complaints if you violate these rules, and it will make it harder for the lab to conduct research in the future (reputation matters!). Therefore, it would serve you well to learn about mTurk culture before posting studies.
As a first pass, Dynamo Wiki provides indispensable guidelines for academic requesters and helpful tips for being a good Requester.