Neverwinter returns in a better story and with the same modding potential that drove the old NWN community for years.
Also, gone are the henchmen of yesteryear. You will now find your character traveling with a number of interesting companions, all of whom have unique stories and personalities. Doing or saying certain things can affect your influence with your companions, determining what they are willing to tell you and, ultimately, whether they stay or go. This is not necessarily as effective as it sounds, however, because gaining influence with companion characters amounts to telling them what they want to hear, as opposed to (in most cases) being able to actually change their minds about something. Still, it's a nice feature and a welcome addition. Perhaps future updates or expansions will refine it further.
In general, the core gameplay is unchanged from the old Neverwinter Nights.
The audio of NWN 2, while passable, is disappointing in that it recycles many sounds from the original game. Many of the voices are re-used. Some of the acting is questionable, but most of the major characters are voiced very well. From a hardware perspective, the game has some issues with sound (for example, running the toolset killed my sound in-game until performing a reboot), but most problems can be solved by turning acceleration down to Standard or Basic in DirectX Diag. An authentic Creative Labs product will probably run fine, though it's regrettable the hardware support is not more inclusive.
Graphically, NWN 2 features a new, realistic outdoor terrain system and modest improvements in the textures and animation from the original NWN. At face value, the game looks fine, but the performance requirements are so drastically higher than the first NWN, one can't help but wonder what happened to so de-optimize the engine. Nonetheless, a reasonably powerful machine should have no problem. My dual core 4200+ and GeForce 7600 GT experience minor lag only when a certain web spell is cast by wizards and sorcerers. I have played through the campaign one and a half times thus far, and I've yet to see any significant performance problems.
What actually made the original NWN into such a long-lasting game was the toolset -- the program with which players can create their own levels, stories, items, etc. The new toolset for NWN 2 is similar in functionality, and I believe it will provide years' worth of new content, but it is harder to use than the old one. The interface isn't as intuitive, and the learning curve is steeper. However, you can do a lot more with the new editor than with the old one. As the scripting language for NWN 2 (NWN Script) is unchanged from the original game, anyone with previous toolset experience should make the transition nicely. For newcomers, an active forum community and numerous online tutorials should get you started.
Neverwinter Nights 2 is one of the few games to come out in recent years that I would say is actually worth $49.99. Numerous titles are worthy of plucking from the bargain bin or waiting for a price drop to a more reasonable $29.99. But NWN 2's toolset alone makes it a deal, not to mention the freely downloadable content made available by players and builders around the world. Further, multiplayer servers are active constantly, and some aspiring mod-makers have even gone as far as creating persistent worlds for the game (which wasn't even intended by developer Obsidian).
Overall, even if you're like me and are bored to tears with most D&D crap, Neverwinter Nights 2 is a great PC game with a great story. Combined with the toolset, it should be around as long as players want it to be.