So, is it really worth buying Diablo II?  I mean, when you get right down to it, Diablo I is still an awesome game, and there are parts of it that I still prefer over D2.  And if you haven't played it, it might seem like virtually the same game (but it definitely isn't).  Well anyway, here're some major differences you can expect.

 

First of all, the quality is better.  It's noticeable as soon as you start the game -- the entire environment is rendered in 3-D perspective by default option (you can turn it off later if you want).  The graphics are just better.  It gets dark at night and light in the day, and it even storms sometimes.  Spells are really quite spiffy to look at.
There are five different character classes instead of three -- Amazon, Barbarian, Necromancer, Paladin, and Sorceress.  Each one has completely different skills and spells they can learn and requires different tactics.
Instead of playing in dungeons and caves, you play in vast wildernesses and deserts and so on.  It would be incredibly easy to get lost...
...except that there are regularly placed Waypoints.  These function like teleporter pads -- when you find one (step on it to activate it), click on it to bring up a window where you can choose to teleport to another Waypoint.  There's always one in town, so if you're near a Waypoint and run into trouble (a swarm of Fetishes, for example), you can dash over to it and use it to zap yourself back to town, instead of opening a portal.
You also get to play in and around four different towns instead of just one.
A complaint I had was that you can't regularly save the game.  The 'save' function is tied in with the 'exit' function, so saving your game also exits.  And when you reload, your character is automatically placed back in town, instead of where you left off.  So you're often forced to go Waypoint-hopping -- trying to find the next Waypoint because you don't want to give up on all the ground you've gained.
Shrines are set up differently.  There are only a few different types, and their names are self-explanatory -- a Stamina shrine would give you unlimited stamina for a minute or two, an Experience shrine gives you more experience per kill, etc.
Quests are more elaborate, fit in more with the storyline, and take more effort and time to complete.
There are what are called 'socketed' items...items that can be modified by placing up to three gems in them.  Different types and qualities of gems have different effects.
Rare items are like magic items, except they can have from three to five magical properties, instead of one or two.
Item sets are groups of matched special unique items.  When you have collected all of the items in a set and have them all equipped, you receive extra magic benefits.
Your single player characters can be played over b.net.
You can create characters on one of b.net's Realms.  Realm characters are stored on Blizzard's servers and can be played from any computer.  If, for example, you have a Realm character on USEast, you can only play against other USEast characters.