If you had not played Fallout 3 you would think it was good.
So storywise its pretty good i think, gameplay is like fallout 3. However they didnt really expand much on what they had, typical obsidian. You have companions who dont have really much to say after you talk to them once they dont change as you further the game which i thought was boring basically its a "Hiya i has a quest for you" so you do the quest then they just say them same things.
Theres a huge new crafting system which was ambitious but to be honest i hate all that crafting crap. Collecting stuff once to make a gun is ok but for food or ammo...its just not worth doing. I didnt do any of it and i manage along find. I think its a waste of time.
There are new and old factions come back up in vegas personally i was disappointed not to see Enclave as they are the only real faction to match the Brotherhood of Steel with technology. There isnt many armors in the game and this is evident with only 2 diffrent looks for power armor and IMO they look crap compared to fallout 3. Theres loads of new guns though and thats a good improvement.
All in all its a good game just lacks alot of things that could have made it epic. There are niggling things in the game which will annoy fallout 3 like the lack of armor mentioned earlier and the general feel of the game is diffrent I would say fallout 3 was far more hostile in terms of raiders and such. Vegas is like the rebuilding of civilisation rather than a fight for survival in fallout 3.