Deathclaws are a helluva lot nastier in NV than they were in 3, that's for sure. In 3, I could wander into that town infested by Deathclaws with an Assault Rifle and my trusty T-51b Power Armor, with Dogmeat as my only companion, and clear the place no problem. In Vegas, I can wander into the Quarry with Enclave Armor and an Anti-Material Rifle, with Boone and Rex, and it's still a gamble if I can take down all the Deathclaws before they rip me to shreds. Thankfully, they seem to be the only Deathclaws in the game, other than the Legendary one.
The Strip was not what I expected (I expected the actual Vegas to be bigger), but it was fun, for what it was worth. I've broken the bank at all the casinos and finished all the side quests for Vegas (in my next playthrough, it's gonna be a strict job for the Legion, since I've done everything else; I can use this playthrough for both the NCR and Yes-Man). I did have a huge glitch in the Ultra Luxe when doing the "Beyond the Beef" Quest; At the part where you expose Mortimer and they stop attacking you… well, they did, then started attacking me again. I had to flee the casino like a madman, and Heck Gunderson got stuck in the kitchen when I was fleeing. Weird thing is, when I came back to the Ultra Luxe a few days later (game time), he was with his dad and the Quest showed as completed. What the heck? I can still go in the Ultra Luxe, but if I go in the Cafe past Marjorie, any White Glove members that see me will instantly be aggressive, even though I'm Idolized with them. Well... whatever.
My favorite part of Fallout (any fallout) is the Vaults. Sadly, the variety of the vaults is lacking in New Vegas. 3 had the best vaults, in my opinon. The Gary Vault was the fucking creepiest thing ever (I don't care what anyone says, wandering around a vault when you hear a bunch of guys going, "Gary..." every minute or so is scary shit), the Mindfuck Vault was fun (the one where you were hallucinating), and the VR Vault was fun, too. Not to mention the FEV Vault where you get the GECK. The only lackluster vault in 3 seemed to be the Music one (the one with all the Mirelurks), although blowing up the Mirelurks with the Sound emitter was fun.
In NV, however, the vaults were generic, except for one notable exception. The Fiend vault was dull; there was no creepiness factor I expect in a Vault Tec Vault; The Plant Vault was actually a generic concept; although it was a good idea, it has been done too many times before to be interesting. The Super Radioactive vault with all the weapons (Vault 34) is a good survival vault, but seemed too straightforward (Feral Ghouls + radiation = race against time), and Vault 21 was lame because you couldn't explore it due to the lower levels being sealed up by Mr. House. However, I did enjoy the Political Vault for some reason, as it was kind of a murder mystery. The Sacrificial Chamber at the end was a neat touch, too. Plus, there was only four skeletons at the entrance when five survived, meaning the fifth person is out there... somewhere. Maybe in an expansion?
The Legion as the "bad guys" doesn't feel as creepy as the Enclave. Enclave guys in their mantis-head power armor was fucking creepy as shit; Guys fighting in Roman Battle Armor with AK-47's and fists just doesn't have the same feel. At least the NCR's Elite Rangers has those breathing masks, but... it's still not the same.
Super Mutants also have a different feel in the game. Most of the actual Super Mutants are good guys, anyway; the only bad ones seem to be the Nightkin, who just don't have the same appeal. Maybe it's because, except for the ones in the Repconn basement, you're fighting them outside, on a mountain. It was a different feel wandering through DC ruins, never knowing if Super Mutants are going to appear or not when you turn that next corner.
Maybe it's just me, but it seemed a lot more desolate and bleak in 3, since the Capital Wasteland was really that... a Wasteland. I'm not saying that the Mojave Wasteland is Sunshine and Rainbows, but... it seems a little more optimistic.