You know a game is good when you begin to compare every RPG you play to it, as I have for Diablo II.
I must have played this game though 20 times and I can't tell you exactly why you're going after Diablo except for the fact that he's the devil, and its better off with the devil dead than running around your neck of the woods.
Each class has three unique and highly specialized skill trees, which you can upgrade as you level up. These trees are not all balanced, and you will quickly find that some skills are useless compared to others, and will lean heavily on just a handful of them. The gameplay is fast-paced and rewarding especially when playing online with others, and the bosses (especially Duriel) are challenging.
PvP is almost a joke, considering that end games you maybe have a couple thousand health and can easily do thousands of damage in a matter of seconds. As a result, PvP damage has to be drastically cut to make the fights longer than 1 second. But PvP is almost irrelevant.
I have a few problems with the game. Firstly, the most fun I had playing the game was being a necromancer, having a dozen summoned skeletons fight for me. However, bosses do extra damage against summons, meaning Diablo one-shot killed all of them at once, making a summing necromancer an impossible class to solo bosses with. Also the stat system that you use at level-up is flawed. Each level you have 5 points to spend in things like strength and health. The problem is that strength doesn't affect your damage significantly enough to be more useful than health, and dexterity which affects how often your attacks hit an enemy is completely broken. You need an insane amount of dex to keep your hit rating at 95%, unlit ultimately you find a weapon which ignores defense and you need 0 dex, making all the points you put in there originally a waste. Also, the skills you level up cannot be unlearned and replaced, meaning if you make a mistake you're stuck with it until you make a new character.
Diablo II offers countless hours of fun to any RPG fan despite its rather ancient-looking graphics, co-op play is fun, and it will be impossible for you to only play through the game once.