Well, at it's core, its just a $50 expansion to fallout 3. They took the same engine and looks, and made a new adventure with them. But, it's got a lot more stuff to offer than 3, and has plenty of issues fixed, such as there being no bobbleheads. The game is balanced much more, so you can actually have a challenge at any point in the game. Though, there are plenty of bugs, but they're getting fixed.
No, gambling is not mandatory. The story is pretty good, there are four paths to choose from, Evil (Ceasar), Neutral (NCR, from what i can tell), Good (House) and whatever the hell you want (Independent). I haven't finished it yet, but from what I can tell, all four paths with eventually take you to the same ending (somewhat), but the outcome may differ depending on what you chose. I would suggest getting it. There is plenty of open space in New Vegas that may be filled in at a later date with expansions.