For a lot of people, that won't be a bad thing -- New Vegas doesn't break what's not broken, and the few things it does change are definitely improvements. VATS is updated a little bit to make activating it smoother, and add a few cinematic views to the gory proceedings (a new "kill cam" mode shows you some big kills even during first-person action). Companions now have their own rotary menu with eight different commands, so you don't have to talk to them every time you need them to do something. Books have been changed into magazines, weapons react a little better, and we've heard that guns can be modified (though I didn't get to see that during the demo).
But despite the differences, Fallout: New Vegas plays the same as the first game. Story is relayed through lots and lots of dialogue (with the same old Bethesda zoom-in when you first meet a character), and the game has you exploring, shooting, talking, and doing more exploring.
I started out on the game's equivalent of the strip, and while I was disappointed to see that most of modern Vegas' landmarks aren't there, the dev explained that this is of course a fictional version of Vegas, set in the world of Fallout. To that end, they created all of their own casinos, from palaces like Tops and Gomorra to a dive called Vault 21 and the Lucky 38, a casino closed off to anyone but Ghouls. I stopped at the first one I saw, and headed in to do some gambling.
I was forced to check my weapons in at the door, but of course, that was a choice -- it was possible, through the dialogue tree, for me to keep a concealed weapon, or even fight the bouncer, but I decided to try my luck at the table rather than in combat. Inside, I found three games to play in the form of blackjack, roulette, and the slot machines. It's all done as you'd expect -- that same retro font from the last game lets you place bets, and you try to hit it big along with all of the other poor schlubs with more money than sense.
You can't hit it too big, however -- the dev showed me a demo build cheat to win the slot machine jackpot every time, and as soon as I did, the casino's goons showed up to end my gambling career. I was also told that there was one more game, created for Fallout, that they weren't showing at this demo, and as I left the casino, I noticed there was also a show to see, as well as the usual gathering of sidequest-worthy locals hanging around.
As the dev warped me to the next adventure, I asked if he thought this game was an expansion or a sequel, and he told me that it was definitely not just an expansion. The world is comparable in size to the area we've explored around DC, and there are lots and lots of new models and new art to work with, he said. But I'm not sure I agree -- new content, even if there's lots of it, doesn't necessarily mean a new game.
The few quests I played put me in the middle of a battle between bases of the New California Republic and the Caesar's Legion faction, and all the old tricks were there -- I scouted for the NCR, ran an assault with a few helpers in tow, and could choose multiple ways to take down my enemies (including dressing up in Caesar's Legion gear and sneaking into the base). New drugs will give new buffs, the PipBoy still manages everything (including new radio stations, though I wasn't able to hear any during the demo), and the denizens of the desert are just as crazy as what we found in the DC underground.
In short, if you loved Fallout 3, you'll probably love New Vegas just as much. But if Fallout 3 didn't kickstart your GECK, the new setting doesn't seem like it changes the gameplay all that much. War, I believe they say. War never changes.