D3 is dull because its single player.
D2 is amazing because its a lot of fun and holds a playerbase without Blizzard adding any major new content since 2000. 16 years! How can it DO that? Doesn't that fly in the face of everything developers say when they assert that you need to add new content to keep players from getting bored? The reason D2 can do that is because it is social. You have a chatroom lobby. You have big 8 player games where players interact. Probably the most addicting aspect is the early game where there's a lot of advantages in grouping up to clear tristram or clear act 1 or do tomb runs.
D2 proves beyond a shadow of a doubt that you don't need new content at all. You need a multiplayer experience. You need strong social tools.
Then you look at WoW and they add new content and revamp old content like CRAZY and the playerbase keeps shrinking. But that's because they murdered the in-game community, restricted and cut off in game chat, and made it as antisocial as possible. They go oh look MoP is amazing the raids are great the content is polished and NO-ONE plays?!?!? How can that be? Because you don't talk to anyone ever!