My character is a good guy. He likes justice, law and order, asking questions first and shooting later, all that stuff. He also isn't afraid to gun down an enemy. Are there any factions that he really shouldn't be sided with? He's already made an enemy of the Powder Gangers.

Have you been to Nipton yet?

I think you will find it fairly obvious who you won't want to side with, so I won't spoilerize it.

Yeah, my first time through Nipton was a no-brainer for me, too.

Looking forward to hearing about your experience there, Mandaroff!

Part of playing as a character is to react as your character would, which foreknowledge kind of spoils, but as you asked - in addition to the Powder Gangers, there's the Omertas, Fiends naturally will be shot on sight, and arguably the Great Khans. NCR will be a natural fit on the surface, but if you value order, don't forget the Legion.