The interplay between factions is a key gameplay element in FNV. Aligning yourself with one group will engender the wrath of others. The more you attack a faction, the more aggressive its members will become towards you - to the point where certain quests will end up locked off, as they involve encountering characters who will simply shoot you on sight.
Alternatively you can play the factions off against each other, switching allegiances as and when it suits you. A brand new reputation system has been implemented to keep track of your actions. It's said to be much more complex than the old good and evil karma system, and your choices will have many more and greater effects than they did in the previous game. They will also, of course, determine which of the multiple endings you see.
Companions are back for New Vegas, but the factions come into play here too. In the Mojave desert section I was accompanied by a chap called Boon, who I could command using the companion wheel - telling him whether to use ranged or melee combat, making him carry stuff to free up my own inventory and so on. Each companion has their own back-story and you can talk to them about their past. Turns out Boon is an ex-New California Republican who still has sympathies with the cause, so you'd better not take out too many of his former associates if you want to keep him onside.
There wasn't enough time to fully explore the interplay between factions during this short demo, so I settled for mucking about with some of the new weapons being introduced in New Vegas. These include long-range additions such as spears, which can be used in combination with the VATS system to perform some of the most satisfying headshots you'll ever pull off. Then there are incendiary grenades, which not only set enemies on fire but send them flying into the air.
New melee weapons include a Wolverine-style metal gauntlet and a nine-iron golf club. They now have secondary attacks - select the 'Fore' move while wielding the nine-iron, for example, and your stroke will pack even more of a punch. Meanwhile, the gauntlet can be used to deliver low-down and dirty hits as well as powerful uppercuts.
So then: enhanced weapons, more useful companions, a new factions system, cheerier visuals... In short, New Vegas looks set to be more of an evolution of the Fallout series than a revolution. Not that fans of the previous game will be complaining, of course, and not to discount these changes and new features. Judging by what we've seen so far, they've been conscientiously thought through, and sit comfortably within the framework which made Fallout 3 such a success. This may not be Fallout as you've never seen it before, but it's certainly Vegas as you've never seen it, and exploring this world looks like being an awful lot of fun.
Fallout: New Vegas is due out for PC, PS3 and Xbox 360 on 22nd October in Europe.