MrTony,
Obviously, we would want a fan who is intimately familiar with a movie who has a LOT of insider info on how certain shots were filmed, or who can point out filming flubs and mistakes, etc.
The creator of
http://www.thegoonies.com/ could probably record a pretty good commentary of the movie The Goonies.
I could record a commentary for the last episode of Quantum Leap, but it would be mostly negative. I'd be pointing out how many mistakes they made in it.