Totally. A lot of traditional MMO mechanics really don't translate into persistent world gaming outside of the heavily defined RPG genre.

I think the only way to make a game like GTA work as an MMO would be to create in-game consequences for actions. As a thug, you start shooting in the street, suddenly all the (player) cops are given a green light to go take you out. In other words, if you could reward thugs for being covert and cops for being aggressive, you'd have an always-tense, slowly boiling city of potential crime and corruption.

But yes, it would require quite a bit of redefining.