Most of it boils down to trust.

The main problems of these systems are that if you can get $3/hr of economic value from a single turk, it's hard to trust that you are getting accurate results without checking by repetition. This means that you could end up repeating the task 2, 3 times or more to get accuracy.

Scammers and bad workers are a big issue on any platform like this. I'm not sure how easily this can be solved - the workers only incentive to not cheat the system is the possibility of the requester rejecting their work and them not getting paid as well damaging their rating.

Whilst this seems like a good-enough way, there are enough lazy / bad requesters who don't bother rejecting work which skews ratings of workers. Also, ratings on mturk are lifetime ratings, rather than last n days, which isn't that indicative of recent worker quality.

I'd argue the solution to trust is appropriately matching work to reliable workers for whom the work is fairly paid, not simply raw repetition.

At $3 an hour, someone in (say) Lithuania would be highly motivated to do the work right, even if someone in the US wouldn't particularly care.

This is the approach we took at MobileWorks, and it works fairly well.

A possible solution could be for users to have an accuracy rating (perhaps short-term and long-term), and for the Turk API to take an optional minimum accuracy rating for tasks to be visible to a given user. (For all I know this is already in place; I'm not a Turk user.)

I would almost argue there is a digital food chain.For example by moving from MTurk towards Odesk towards VWorker towards Elance and so on. It really really all comes down to the individual skill level and with the wealth of information available online it comes all down towards the level of dedication one is willing to invest into acquiring a marketable skill.

The problem I see with these articles is, that they try to paint MTurk in a bad light and I would hate to see it disappear just because it becomes an image problem for Amazon.

Economic value is nothing more than what someone will pay. It is meaningless to say that if people will only pay $3 then there's no way anyone should pay $10.

On slave-owning plantations, the owners were paying $0/hr in wages, but that didn't mean that there was no way slaves can or should be paid more than $0/hr.

Once a work system is good enough to provide 5 minute highly qualified answers to questions like "is this CT indicative of cancer?", people would be paid $3/hr for those answers.

Scarcity and demand, not ability determines wages.

I agree that ability doesn't determine wages. But I would like to qualify your statement about what does determine wages. In a free market, wages are determined by scarcity and demand. However, a free market is a theoretical construction that often fails to model reality. For example, free markets do not include coercion, deception or collusion, all of which are ancient and universal parts of human life.

For example, the $0 pay rate of a field slave is not because nobody wants cotton-picking, or because there are too many people who will pick cotton for free.

Indeed. That's why equating pay to ability is pretty silly.