The Examiner, well, examines the primary reasons why Diablo II continues to be so popular nearly a decade after its release. Two of them:
Interesting items and Item customization
Diablo II didn't invent the idea of customizing magic items, but it may have perfected it with the Lord of Destruction expansion. The addition of runes and runewords makes it so that the player really feels like they can craft a special magic item, and the system remains simple without removing the difficulty of obtaining the best runewords.
Rogue-like random dungeons and land masses
The quality that most Diablo-clones miss out on is the rogue-like random dungeons and land masses that make playing Diablo again and again much more bearable when you aren't running through the exact same dungeon over and over. While Diablo II can be considered very light in the Rogue-like qualities -- it's really as much of a re-arrangement of the areas as opposed to real random areas -- it certainly beats the extremely linear competition.