Some thoughts off the top of my head on the MDC. These probably apply to many similar challenges by skeptic clubs:
-Overall, IMO it is interesting for reasons of entertainment and getting the 'skeptical good news' out there. But it is not a good platform for Science, except at the most rudimentary 'doing proper experiments is good' level.
-Despite some lip service, such challenges hardly seem unbiased. We typically have a challenge by a club ran by someone(s) who is obviously openly hostile to claims he says he is interested in investigating.
-If one was really honest about testing the phenomena and being impartial, they'd ask you once to take the challenge then if you decline they'd leave you alone. Instead, we get constant bullying to take the challenge, and these 'I wonder why...' and 'probably hiding under a rock' and 'they don't have a use for the money?' comments and cricket sound stuff.
-The mere fact that everyone is entitled to persue the standard channels of Science. Standard channels of science would be testing by actual scientists, peer review, submitting journal articles, presenting at conferences, folliowing the evidence wherever it leads and whatever topics, etc. Standard channels of science are not being hounded by skeptical clubs. Will standard channels of science make you sign a contract? Make you agree that they can use all media of the test (which will probably go into hostile commentary)? Only focus on the big media guys instead of all claims? Make funny cartoons of you? Of course not.
-One challenge won't mean much in terms of science, even many well-known people in the skeptical movement admit that. Science is done by replication.
-If a person passes a challenge, it could be due to chance. If enough people apply to do probability based tests, we'd expect one of them to win by chance, given a probability p of passing the tests and given enough tests.
-The logical fact that if the supernatural/paranormal happened that it would suddenly become a natural/normal thing. A supernatural/paranormal challenge would no longer apply. Even if the money is paid out, for the logical reason discussed it wouldn't mean much.
-Statistics from past tests not able to be obtained easily. It is difficult, it is made difficult one could say, for any interested person, let alone an applicant, to easily see statistical summaries on past tests, broken down by various categories such as type of test, number of successes out of number of trials, and probability of success per each trial.
-Intentionally useless provoking questions to potential applicants like 'why don't you win and give the money to charity?'. The testers can just as easily give their millions to charity too, or at least the interest it gains every year, so I always have a hard time when that question is asked. If the donators didn't donate their money with that in mind, which is the usual response, I'm sure they won't mind donating to charity because they are such nice people. To summarize: these provoking questions get us nowhere. Why? Because it has nothing to do with science. It does, however, have everything to do with playing 'gotcha!' with someone you consider an 'opponent'.
-The history of the challenge shows that it did not start as an open-minded scientific quest to test so-called paranormal things, which is what is implied when one states things like 'I'd love for paranormal things to be true'. It started as a challenge. From wikipedia: "In 1964, when challenged by a medium to 'put his money where his mouth is', Randi put up $1,000 of his own money to the first person who could provide objective proof of the paranormal."
-A founding member of CSICOP, Dennis Rawlings, states that Randi said "I always have an out." in reference to the challenge. The full quote according to Randi is "I always have an out, I'm right!". This doesn't really change things. It is evidence of a really strange offer. You're offering people money to win something you know they cannot win? What is the point again? Is it to expose, to get popularity, or to genually test for so-called paranormal things?
-The testers could have been fooled, which anyone would admit is always more probable than supernatural/paranormal anything existing. More on this with the following remote viewing challenge given by Randi in one of his commentaries (bolding mine):
A few clever people familiar with cryptography correctly decoded Randi's code. See
http://www.crypto.com/blog/psychic_cryptanalysis/. What if instead of some math geeks that this was done by the people Randi challenged? Randi did say "all they have to do" to win the money. Would he hand over the money if they revealed the contents? No, of course not. There would be more hoops to jump through due to contract, as well as it being more probable that some type of trickery occcured.
Especially note the part they write about the encoding being able to be interpreted many ways. Is this an "I always have an out" stuff, that is an intentional way of making something unwinnable, or just an honest oversight by Randi? The problem is, one cannot tell.
-Claims that such organizations are educational. Maybe so, but when was the last time any educational organizations was making fun of people in weekly commentaries? It just doesn't seem proper or helpful.