Diablo II is quite simply a blast to play. It improves upon the original in everyway... better story, graphics, gameplay and multiplayer all come with Diablo II and, most importantly to me, this game was LONGER and had many more class choices than did its predecessor.
Character classes are much more specialized now (no longer can a Warrior cast high level spells, just as a Mage can't use as high level of weapons as he did before... although the magic class is a sorceress, not a sorceror, as before). As you gain experience, not only do you get to add on to your regular abilities (Strength, Dexterity, etc), but you also get to add on to your abilities specific to your class! Each class has 3 trees of skills to choose from, making many different builds for characters possible.
Each build comes with a unique strategy of playing the class. Players will have to adapt to survive.
This game is a classic, playable in parties or by singletons, one that every Action RPG fan should check out.