ROFL
As far as factions, I don't think they have anything to do with grinding, I think how you approach the mission is what will affect your reputation. The example they gave is if you kill the Marine guard guarding the embassy to get in, you will be on the Marines' crap list, and they will fight you once you get inside, otherwise they help you. In another example, if you don't kill the arms dealer, you probably lose rep with whoever gave you the assignment.