This game came out around the 2nd half of my college life. If it wasn't for this game, along with starcraft, I think I would've graduated one year earlier.
Anyways, Diablo 2 is an action adventure game. You go around killing monsters to level up and get nice items. Everytime you level up, you get skill pts and you get to decide which skill to learn. Because of this skill system, you can make your character very unique but thats not usually the case because theres usually a few optimal ways to go. This changes every few months though, at least back then, because blizzard would keep updating the skills by either nerfing(weakening) or buffing it. For instance, the sorceress at one time was strongest and killed enemies the quickest with the frozen orb build. But eventually blizzard nerfed this skill and it became pretty weak so everyone went with the new build of the month, firewall. But the best part of the customization of the character really came from the items more than skills in my opinion. Their loot system is the best in any game out there.
Everyone in the game, at least before the expansion Lord of Destruction, was after yellow/rare items. Rare items had random modifiers and you could have some totally useless item or some godly item depending on your luck. Because of this system, everytime a yellow item dropped you get that rush of adrenaline crossing your fingers hoping you got one of the good items but 99% of the time you got a dud. This game was so addicting that people were just doing the same thing over and over again for 8 hr straight hoping a nice item would drop for them. Of course after the expansion, they somewhat killed the game in my opinion. They totally nerfed the rare items and all people wanted were the gold/unique items (which aren't random stats) or the runes that you use to create a magical item by combining them. Some people might enjoy this, but i felt it killed the game.
In the end, this game ended up being a classic and it only makes Diablo 3 one of the most anticipated games coming out.
Pros: -Skill tree system that you can customize your character to your style/preference (even though there's one optimal build but no one has the exact build though) -Best loot system out there -PvP is fun. Go into a game and just try and kill everyone to piss em off, or vice versa