The Baron said:...
1. Possible bias of the reviewer of the review?
As long as the case is convincingly made and proven, bias becomes not only unimportant, but invisible, IMO. Bias is most apparent when you support points you either fail to justify, or don't even attempt to justify.
2. The sheer amount of time it would take for one person to read every review (or most reviews) of a particular card and then compare those reviews against the reviews of others
Well, I don't think you'd have to do that as often many reviews simply copy the bias and errors of an earlier review done elsewhere, so that if you can trace down what you consider to be "the review" on which the others hinge, you can address most of them by addressing that review.
3. How long before some site says you're stealing their content and then decides to go after you
Stealing content = reproducing it as though you originated it
Doing a critique of a review in which the important material is your own would not be stealing material.
Dunno, really. It's a good idea, but I don't think it would beat out the buyer reading several reviews himself in order to make a decision. Nor would it replace the boards that provide feedback for the mistakes in reviews (Brent and UT2003, Anand's 5900 review, etc.).
Actually, this kind of thing is being done and has been done for a long time. It really isn't necessary to quote someone else's review to do a critique on it--you need only restate the flawed concepts in the review you wish to rectify. Most often the material addressed by the critique need not be directly attributed for your readership to ascertain its source.