There are various clusters of homogeneous people. You have your classic /r/beermoney students, same as you would get from regular studies on campus. Then you get your bored housewives of US, and oversees workers mostly Indians/Philipinos.
Once you realize these large groups of people exist, you can still get useful work done by accounting for obvious biases.
Disclaimer: I've used MTurk both for surveys as a requester and also used it as a Turker for a month two years ago, whenever I got bored.
I saw so many Kahneman inspired studies it was not even funny at the end. (Monty Hall problems, priming, $1.10 bat+ball problem, etc etc) . I ended up making around $6 an hour.
The best problems were porno classification problems.
I suppose that is standard stuff you do in psychology these days.
I haven't actually looked at the demographic information we've collected. Most of the work the group does has to do with memory and engagement, which varies enough person-to-person that their demographic doesn't matter so much.
