D2 had some amazingly easy gameplay. Had a good replay factor, along with good graphics at the time. It was possible to solo most places(if you gear was good enough), mob scaling for party members if I remember correctly so you couldn't completely cheap it out. A fun loot system, easy to navigate world, and even a decent pvp mix.
I think a lot of the appeal to replayability was in the randomly generated maps. Of course some areas were static, but most areas were randomized enough so it wasn't a pixel for pixel copy of the same cave as last time. The challenges were enough to get you frustrated by eventually figure out without resorting to some step-by-step internet instructions.
The open trade system was very easy "2 sojo, pst"

, and the generic items had enough random stats to make even blue items worth while gear. Heck some blue items beat out uniques for particular purposes. Gambling was a nice way to toss away money for a chance at something good. Crafting and sockets was a bit much but hey they all can't be gems I suppose.
I still say that pre 1.09 was the pinnacle of D2, and no game out will touch it in the aspects that I've mentioned. I played D2 for I want to say close to 2 years, and even with an 8 player cap per "game" it was still one of the best titles that I can think of for the hack and slash genre.