In FO 1 and 2, you could (though you have to have some unofficial mods to fix the problems with many quests) complete without firing a shot and making everyone like each other (or at least tolerate each other and be better for it) making you like Ghandi of the Wasteland. But New Vegas doesn't allow this. You have to choose. You have what essentially boils down to four choices: NCR, Independance, Legion, or Yourself. You can not, for instance, get both the Legion and NCR to back down and come to some sort of peace agreement, even if it would just be who controls what side of the Colorado. It's not like Caesar is not a reasonable man. He is pretty intelligent, and if you could-through a show of force obviously-convince him that it would be wiser to have NCR as an ally, they could seem to be a separate part of his army, but they would be like a dog on a leash (or so you should be able to tell him).
If you're persuasive enough, you should be able to appeal to everyone and make them see the bigger picture in the end. If they are reasonable. True, there could be some fringe groups out there, led by a bullheaded idiot that will NEVER join your cause and they would have to be dealt with; but they would die out with that kind of leadership in the end anyway, without interference from anyone else. Being vague on purpose, there is already a group in the game that most people want to destroy and are bullheaded enough to make you want to agree with them; but there IS a completely diplomatic solution around them (though for some it is bugged) so why couldn't that extend deeper?
That isn't to say I don't thoroughly enjoy the story of the game; it's pretty damn impressive. It's got all the colors of a monochromatic rainbow except for the big one I am talking about here, it's full of humor, and it's really, really well thought out. Maybe it's just to appeal to a larger market where there has to be a clear evil. The Legion is clearly "evil" and they don't just threaten the NCR, but every group in the Mojave. Traditionally, stories like this usually have some hidden meaning or moral lesson to be learned, but in the past the Fallout games' moral lessons were much more profound, and deeper, and took much thought to fully understand the wisdom in them. Is this a bad thing? No, not necessarily. I just was expecting that same sort of unique story telling I had come to love in the original games and not a good, but "mass-marketed" plot.