I will probably be playing DIII for a long time - I just hope its better then DII. To be honest, I do not have high hopes for DIII, but you never know.
Diablo - great game that set a standard that all others were judged.
Diablo II + lord of destruction expansion - was a good game, but lacked a lot of stuff, such as a large amount of side quest, limited NPC interaction, little story plot - besides the cut scenes. The portals seemed to "patch" the world together. Poor graphics for the time - while most games were running at 1024X768, DIII was 800x600. With DII the player had to follow an exact path everytime they played the game; after the first hundred times through, this got a little boring.
Diablo III - ????
There were a lot of things that made the original diablo a great game. While at the same time there were a lot of things that made DII a good game, but not great.
I hope that DIII follows the example set by the original diablo.