Concerning demo: I agree with not making the demo compatible with the retail game, but as far as the standard mutators go, some of them alter the game in such a big way that people that are thinking of getting the game with these mutators in mind, will not be able to get an accurate feel of what they're hoping to play.
Concerning instagib: There is no debate as to whether it is a gametype or a mutator, as this is not a matter of opinion. Instagib is a mutator, and yes, iCTF is played in an entirely different manner than normal CTF, but that does not change the fact that instagib is a mutator. Even though I really enjoy iCTF, I thought it was a bit strange for Epic to call it a gametype in 2k4, because it doesn't really change anything. (read: change as in setting it up and/or filtering, effort wise, not talking about its effect in game)
Concerning the topic: haha, next!