Okay, let's get one thing out of the way right now this game is not "just an expansion pack." The definition of an expansion pack is something that builds upon the original game in some fashion. Fallout: New Vegas is an entirely new story, totally unconnected from the story we experienced in Fallout 3. It brings a large new map, various pieces of new equipment and a host of other improvements to the table.
Let it never be said that New Vegas starts off slow. This game, quite literally, starts off with a bang. A bang that involves a bullet being put in your brain pan. Shortly after being patched up by a small town doctor you, a nameless Wasteland Courier, are unleashed upon the Mojave Wasteland to find out why these men attempted to kill you, what exactly they stole from you and why. Of course this simple goal spirals out of control until you're caught up in the middle of pitched, life and death, battles between the various factions in the Wasteland.
These factions are one of the main improvements from Fallout 3 to New Vegas. While you could work for various groups in Fallout 3 it had little bearing upon the way other groups looked at you except in very pre-scripted ways. New Vegas brings in a Reputation system that guides how the many different groups around the Wasteland view you and your actions.
When playing the game you will come across many missions that require you to pick a side. Want to help out the New California Republic? Well there's a good chance that you're going to have to kill some members of Caesar's Legion during the mission. Obviously this isn't going to make you any friends amongst the Legion so you're going to have to be very careful and pick your enemies fairly early on.
Of course you have to be careful when traveling the wasteland since some of these groups are fairy hostile to you from the get go. You'll either have to run away, sneak past them or get your hands on their clothing so you can disguise yourself as one of them. Of course this isn't fool proof as guards and their dogs can still detect you while disguised so you'll have to use this sparingly. But it can help you get through some nasty areas if you're finding it too hard to fight through the enemies you've made.
At its most basic level New Vegas simply takes what Fallout 3 had already brought to the table and expands upon it to somewhat mixed results.
For example in Fallout 3 you spent much of your time using VATs to avoid the somewhat sloppy first person shooting but that's not as much of an issue here. Using the iron sights really does help with both short and long range shooting. Unfortunately due to how sluggish the aiming is you'll still find yourself forced to use VATs to end most mid to long range encounters.
Another example are the weapon modifications: while a great idea in that letting you modify non-unique weapons lets you make them feel more personal, more yours, it flounders in its accessibility. To put it bluntly it's unlikely you're ever going to use these unless you happen to have a guide or seriously luck out in finding them. You need the luck of the gods to find one for a weapon you're actually using which means they amount to less than nothing for the most part.
All of the improvements really do mean that Fallout: New Vegas is a better game than Fallout 3. Unfortunately New Vegas was released as a glitchy, buggy mess of a game. Normally a title like this would have been reviewed rather promptly after release but various game destroying bugs kept rendering saved games unplayable. This continued to the point where the review had to be shelved. Since that time Bethesda has moved quickly to patch the worst of the glitches although many still remain. However much of this seems like it has to do with an aging game engine that can't handle how much stuff has been crammed into it than bad programming.
As far as graphics and sound go this is basically a retread of Fallout 3. There's a wider variety of graphical assets in the game, such as more varied looking NPC's, and the world isn't quite as drab to look at but overall this looks like the previous game. Sound is of more or less of the same quality except when it comes to the radio stations of all things. The songs just aren't as enjoyable to listen to as before and the deafening silence of the Wasteland is preferable to it which is a bit of a shame.
The one place that Fallout: New Vegas flounders in, that isn't technical, is in the storyline. Fallout 3 had a very personal narrative in that it followed your character from a young age, through finding his father and then on to deciding the fate of the Capital Wasteland. New Vegas' plot dies out after you resolve the issue of your attempted murder. At that point it's just a bunch of side quests until you decide to pick who you're going to side with and help them become rulers of the Mojave. That's literally all there is to this game. It's almost like playing Fallout 3 with one extra faction and no storyline.
When this game is running properly, without falling through the ground, NPC dialogue not triggering properly or enemies running through the floor, then it's a very fun journey. But unfortunately, for too many gamers, the Mojave Wasteland has been tainted by something far worse than radiation: glitches. Between losing ten hours to a corrupted save and then a further twenty hours to the patch this game has gone from a great time to pure frustration. To be frank if you haven't experienced any glitches then you're a lucky person and we really do envy you.