Right now I am playing the game as a "friend" of the NCR. (I also haven't killed anyone in Caesar's Legion because I'm not really against them for whatever reason.) If I was playing as an "enemy" of the NCR, would I still be allowed to accept quests from them? For example, if I walked into Camp McCarran would they all shoot me on sight? When I play through again I don't want to ally myself with the NCR because I don't really like them that much and I want a different experience but if I'm not going to be able to do a lot of the quests, is there any point in being "against" them?

There's someone in some thread who talked about having joined the Powder Gangers (and I presume, vilified by the NCR) and he talked about them only shooting at him. I can tell you that I couldn't complete 'Render unto Caesar' because they had vilified me, and THEN I went and attacked just one more Legionary recruit after I got the mark.

I think you can do NCR quests after you reach New Vegas and do some of the quests there, because then the ambassador will request your presence and offer you a pardon for all of your crimes against NCR. They offered me the same pardon even though I was liked by them. Perhaps THEN you could go back and do some of their quests.

Hope that helps.