Everything one could hope for in a sequel.
Baldur's Gate 2, right from the main screen, seems to surpass the original title in scope. It is essentially the same concept as the original, where a group of adventurers (led by yourself) progress through an epic storyline spanning multiple discs and also featuring a lot of additional side content to be explored. Right off the bat, let me express just how much replay value there is in BG2; there is easily over 100, perhaps even 200 hours of play time should you choose to do everything possible within the game world. There are an even greater number of character classes and proficiencies to choose from this time around, which means you can create an incredibly unique persona to use. All of the old classes are available, such as frontline Fighters and magic-using Wizards, but also include new ones such as Monks who fight without weapons and Barbarians who can enter a powerful rage. These new choices don't change the game terribly much but they can add a new, distinct flavor to the adventure as you play through.
The gameplay is as fun, deep, and addictive as before. Combat is fluid, impressive, and pretty intuitive. New dynamics introduced in the sequel make battles, especially at higher levels, far grander and more complex than was possible in the first game. Outside of the battlefield, towns are extremely well realized; there are thought out districts, areas, and building layouts almost everywhere, and there are an incredible number of NPCs with backstory and character to them. What makes this even more impressive is when this is factored into how huge the game is. Fighting, exploring, and even socializing are all made fun by the thought put into BG2. Controls are good, but the problem of pathfinding (especially in some of the very complex areas of the game) can get frustrating. Like in the original, this is easy to remedy with a bit of attention but is annoying nonetheless.
The pre-rendered visuals for backgrounds are sharper and look better than ever in BG2. The characters look good as well, but the greatest improvement comes in the enemies you'll encounter. Not only are there even more than before, but some are truly visually impressive, such as the dragons a player may encounter during the course of their adventure. The spell effects are terrific for the most part, but some of the older ones have the same look which is a little dated compared to the rest of the game. The music is superb, with a wide array of appropriate tracks for most of the areas in the game. The sounds aren't quite as good as the music, but are still quite good and fit in well. The voice acting - which is quite plentiful - is of very high quality. Some of the sound clips can get annoying after being repeated many times, but generally dialogue and acknowledgements are believable and contribute greatly to the feel of the world.
Overall, Baldur's Gate 2 is probably the most epic and grand adventure to be released on the PC yet. It maintains a consistently high standard of quality over an impressively long period of time, and once you've finished the challenging main quest there's a ton of side content to be explored (some of it even more challenging than the main game). For either the casual player or the hardcore CRPG gamer, this is an excellent choice to play.