The mind suck master of computer addictiveness - just like its predecessor...
Diablo 2 is ridiculously compelling. The combination of scenery changes and character progression will keep your mouse finger clicking for months, if you let it. Multiplayer (with friends, not necessarily online) is great fun, as the various combinations of character type create havoc: Blizzard wisely made it very hard to do one-sided player kills, as the possibilities for (non) co-operation are extensive...
Diablo 2 displays a very interesting characteristic: it's an almost completely skill-less game. All that's really involved in progressing are lots of playing hours and mouse clicking. Once you realise this (after several months), the enjoyment becomes a bit tortuous. As computer entertainment, Diablo 2 is a pinnacle of what the old folks tell you - it's just about the ultimate waste of time.
The Bottom Line
Diablo 2 is best enjoyed as a relaxing way to go adventuring with friends. It's extremely good at what it does, which is keeping the player hooked and excited at a low level of brain activity. The atmosphere is incredible, and as a halfway substitute for gaming and tv watching combined, Diablo 2 is in a class of its own.