Diablo II's excellent multiplayer experience and simple, pleasurable gameplay make up for some of its gaping flaws.
We'll start with how the game changed basic gameplay. This is simple, the classes are now more defined and differ greatly from one another. Each class has a skill tree which determines how it will f*ck up the baddies; do you want way x, y or z? Thankfully, each class works very well to support themselves in single player without getting too screwed over. Then again, there's very little that makes classes like the Paladin recommendable over the classes like Barbarian (who tanks through enemies like they were cake) or the sorceress (who can take out multiple enemies at once). On top of this, because each character is so capable, it makes having a class system fairly useless in multiplayer. For a class system to work properly they need to support one another, but when you have the "everyone is special" approach, it take away from multiplayer. Which takes me to my next point about the class system. It's still not recommendable over Diablo's class system. In the predecessor you choose the mage, the rogue or the warrior, but this only determined you starting stats and your appearance. The game allowed it so that you could evolve the warrior to be a fantastic mage if you felt that you preferred magic after a certain point. This is something that I feel the Diablo II system doesn't really work as well with.
Diablo II on the other hand did some excellent things to change up gameplay. Although not drastic things, they help make the game all the better. That my friend, is variety. When I started Diablo II, I saw the opening area and thought, oh great, its the same sh*thole as Tristram with a new spriteset. I was thoroughly impressed though, when I was transported to the second town where I discovered the variety of terrain and monsters the game had to offer. And not just sprite changing on the monsters either; many you had to fight in an entirely new way.
But the monsters lead me to my final point on where DII has failed. Diablo is all about running around randomized areas filled to the brim with baddies for you chop through like knife through hell's sh*ttiest meanest butter. Because of the sheer amount and calibur of some of the baddies Diablo, and Diablo II's, difficulty can be entirely punishing. Now this is not a bad thing at all, a challenge is excellent. The problem lies in a core game mechanic in DII which is an utter lack of saving. While, it does make sense to make your death be the same in multiplayer as in single, it creates an annoying trend of you having to retrieve all your sh*t with nothing to protect yourself with, and your body is surrounded by a horde of monsters that killed you when you actually had DECENT stuff. And yes I know, "use the stash dumbass", but I shouldn't have to, I should be free to save and load as I please. And thanks to the respawning every time quit the game, it makes each session become a long term commitment. This combination is what takes the great elements that DII has and throws them into the p*ss pond.
Diablo II is still a great game, don't get me wrong, but I'm an advocate for the single players. The one's who don't want to have to worry about PvP and the douchebag teabagging my corpse. As that advocate I feel that DII is a good experience, but not as a roleplaying game, as a multiplayer dungeon crawl.