And the challenges aren't really ...challenges, they're just things that you stumble across during normal play. BLAM! Dead fiend. What's "Rifleman" mean? Whatever. BLAM! BLAM!
This is true, considering that you don't need to actually complete specific challenges to get them. Generally, you just get them if you kill enough enemies of a certain type, kill enough enemies with weapons of a certain type, or things like that. These sort of things tend to happen naturally if you do whatever it takes to complete the "challenge", given enough time. When I hear "challenge", I think of something like killing so many enemies without losing any health or within a certain amount of time, or finding some extremely hard to find item, you know, things that could actually be a challenge to get, as the name would imply. Still, they give experience and sometimes perks, so at least they have some real impact on gameplay, achievements, on the other hand, are, like in most games, just for bragging rights.
Still, I don't hold anything against achievements, I never got why some do, honestly. They're pointless and I don't care about them, but they don't diminish the game in any way, for the same reason why they also don't improve it, they don't effect gameplay at all, it's just something you can brag about to your friends, which really isn't that different from the scores that many older games kept, to be fair, though, a lot of the games like that were the kind where the focus wasn't so much on finishing the game to complete the story, if there was a story or actual way to "finish" it at all, thus, one might argue that in those games, a high score was the only real reward to strive for. But in a game like New Vegas, bragging rights rewards are just decoration.