the plot for Diablo II is that Diablo took over(corrupted) the hero that defeated him the first time and is out to rescue his brothers(Mephisto, Baal) from their various imprisonments and that you must stop him from opening the gates of hell into the mortal realm. Its kinda weak, but ok. there are some good cut scenes in between acts.
The lord of destruction expansion include an extra quest(there are now 5) as well as some different type of items, improved NPCs, and 2 new character classes. I recently broke down and bought the battle chest because my diablo II disc wasnt working so I couldnt install the expansion I already had. So if you want a Lord of Destruction disc just email me or somin giz. I don't play online, and if I do I play open battle.net. all kinds of lamers which i wonder why even bother to get the game hang out there, but i dont like having to be online to play. plus the servers are always getting crashed or hacked.
its not an MMORPG. the single player game is quite good. u just need online play if u like PvP combat or collecting sets or getting a certain item. the style of play is action rpg because u develop ur character and get all sorts of weapons, armor, jewels, etc. but the action and combat is real time. its an awesome game that i definetly reccomend.