Alright, everybody here knows that I'm a blizzard fanboi. I don't even know which of their franchises I love the most. Diablo is a unique game (no wonder the entire genre of neo-graphical roguelikes is based upon that game). What's great about it? The grind is fun. The replayvalue is pretty much infinite. And most importantly, the multiplayer system is very addictive and accessible for everyone.
Of course, as far as mass multiplayer (battle.net) is concerned, I'm talking only about Diablo 2, because Diablo 1, despite being a great game all by itself, and delivering great coop gameplay value and all, fails utterly in terms of Battle.net gaming because of bugs, exploits and lack of features. Diablo 2 is like a half-MMORPG, really: every realm has its rather advanced trading system (sure it's mostly makeshift, but it works), with currency and ebbing/flowing prices; it has ladder that provides challenge and ultimate goal for those who can't just settle for grinding for the sake of grinding (and again, grinding in D2 *is* fun); furthemore, it has permadeath servers which provide the best PvP PK experience I've ever had (sure some say it's annoying and not fun, but hey -- it adds tons of challenge rating and keeps you on your toes all the time, unless you just play private games -- the only problem is the recent outbreak of various haxx that give PKers unfair advantage, but that's mainly due to Blizzard's abandonment of D2 bnet maintenance; in the days of olde, PKing was an art); and the very flexible and multifold item and skill systems that let you experiment with various builds (well of course there's but a fixed number of actually viable (and sometimes imba) builds, but still that number is rather large).
But what is most important, and unique about D2, is the fact that it only but borders the realm of MMORPG, but not belongs to it. Meaning that it's completely free of charge, and most importantly, it has no foolish phantom aspirations to true roleplaying -- instead, it is more like Starcraft -- a highly competitive (ladder) strategy game with high stakes (in hardcore mode). I mean, there even used to be entire TEAMS with sole purpose of being the first who reached the top of the ladder with lvl99 char that season (most notable are team GerBarb and RusBarb).
Hell, D2 was even a little bit of a cybersport game -- even though PvP is generally flawed and imbalanced (blizzard made a disclaimer right from the start: "we are not going to make PvP balanced"), but the 2x2 match-ups were pretty balanced, and very fun and surely not repetitive. I remember there were many 2x2 tournaments held (lanparties usually), in Russia, namely.
Ahh, good ole' times...
If Diablo3 draws upon the core of D2 multiplayer system, its free of charge nature and high competitiveness, then D3 will rock so fuckin' hard.