I'm probably going to get a lot of grief from this, but here goes nothing. I think Diablo 2 lacked a lot on gameplay compared to other Blizzard succes stories (In paticular WoW). I'm not trying to start a huge argument here, I'm just narrating what I've experienced.
Here's what I liked about D2. The different variations of characters (especially after the synergy bonus introduced in 1.10). The very simple skill building process. The extention of the series storyline and the new worlds to go along with them.
Here's what i didn't like. About 80% of the game revolved around killing stuff, while the other 20% revolved around building your character and getting better items so you could kill more stuff. Your interaction with the world was minimal, and MF made many events skipable.
Now... don't get be wrong, I like killing things in games, but eventually that gets boring. D2 v1.10 and its synergy bonus' gave me more ways to kill stuff. Eventually even that got boring due to the attention to character build, which took away from me killing things (And made certain things impossible to kill, ie; you can't kill a lightning immune with a lightning sorc).
What I liked least about the game was Magic Find. It degraded the game by making rare items more accessable, which basically forced players to go on MF runs, or level up as fast as possible to use the items they have found. This completely derailed the storyline by making so many events skipable and bosses easier to access and kill. MF also made PvP a simple matter of who had the best items, and any middle ground a matter of who picked the right skills.
Ultimately, the game was unbalanced.
Now... I'm not saying the game was bad by any means, I have 8 expired lvl 85+ characters to suggest otherwise ( I miss my wolf druid, bowazon, and lightning sorc :usad ). But I am saying that D2, in my opinion, being the staple action/rpg title provided a unique but unbalanced blend of the two genres. D3 should focus on making these aspects balanced and above all, unique. If D3 could combine the fast paced decision making of an action game with the creative inspiration and worldly feeling of an RPG in a balanced scenario.... woahh, talk about hard work... I'd buy it!!!!