I think most of it's just nostalgia. You gotta remember the game is 9 years old and only getting older. Anyway, the game can actually be pretty addicting if you give it a chance. In my opinion, the item system is the best thing about D2.
D2 is the only game that grabbed me and didn't let go for years. It's so addicting once you start getting nice gear, multiple characters/builds, and pvping. I finally let it go about 4-5 years ago, after being hooked for 3.
Once that becomes stale, you join ladder hardcore, and it's a whole different ball game.
Long live v1.09
edit: and I would just like to point out that leveling up is am absolute breeze once you hit level 25. Not saying it's harder before then, but once you hit 25 you can join games where everybody just kills the final boss over and over again. A good game will arrive at/kill the boss in 5 minutes and it's a level or 2 a run every time guaranteed.