Definitely a favorite.
There are two new playable classes, the Druid and the Assassin, and you can take all of your previous Diablo II characters and play through a completely new act in the story.
Story: In the new act, there is a new villain, Baal. Baal is the Lord of Destruction, and the brother of Diablo and Mephisto, and he is bound to take over the wolrdstone. It is your duty to hack n' slash your way through hordes of his minions and eventually, defeat him.
Gameplay: The gameplay remains unchanged, though there are the new character classes. The Druid and Assassin both play completely differently. The Assassin relies on traps and martial arts to defeat her enemies, while the Druid relies on the power of nature. You can also choose from two different weapon sets, so you can have a ranged weapon in one set, and a standard melee weapons in the other, and switching between them is as easy as pressing a button.
Graphics: The most notable change in the expansion is that players can choose whether to play in 680x400 resolution, or the new 800x600 which makes the game look a bit better and makes the interface a lot neater.
Sound: In my opinion, the sound effects are really top notch and entertaining. One of the notable ones are the first enemies you come across, who talk in a raspy high pitched voice, which really suits them. The spells and weapons all sound great when they come in contact with the enemies. All in all the sound is great.
While the single player can be a rather enjoyable experience, this game was meant to be played online with others. The whole gaming experience just quadruples and becomes a lot more competitive and exciting. PVP is great and struggling and gambling to get good loot is always fun. This is definitely one of my favorite games of all time.