RPGame:
I reconsidered D2 a bit, and yes, I do agree that its skill tree and gameplay are good. That DD derives a lot off it, even -- if not lifts several things straight out of it.
But then there's the depth provided by actual side-quests (nope, the obligatory five quests before you defeat the act's boss in D2 don't count as real side-quests, sorry) that have more to them than "kill this! Fetch that! Kill that, and fetch that too!", dialogue, NPCs that have actual personality. Frankly, the thought of replaying this game fills me with dread. Everything next time will be the exact same. No, sorry, I do not consider the "randomized" maps as an aspect of replay value; if anything, I think it's absolutely inane. Oh, look, last time there wasn't a rock here. Now there are two rocks! How exciting! Why, here is a tree where there were three last time. Isn't that stimulating?
I don't value multiplayer all that much, because I get tired of player interaction (one year and a little more with EverQuest was quite enough, thank you). With multiplayer, everything ends up as a race and squabbling for uber loot.