Diablo 2 stands is impressive even when compared to today's games. It expanded and improved on the original quite well.
The gameplay is fairly straight forward. It's an action RPG in the most traditional sense. Left click on the enemy to attack and right click to use what ever spell or skill has been assigned to that button. Enemies usually come in groups and you will find yourself surrounded quite often. Experience is earned for each kill and leveling up allows you to spend points increasing your stats and, much like World of Warcraft's talents, increase skills on a tree specific to your chosen class. Diablo 2's tree is more straightforward and you will find yourself only using a one or two of the skills. I was a little disappointed by this because it meant the gameplay doesn't change much as I went on. Fortunately, the action stays fun throughout, so the repetition doesn't hurt the experience much. The boss battles start off a little weak, but are quite epic by the end of the game. The only one that really bothered me was Diablo himself. He killed me over twenty times, while none of the other bosses came close, including the boss at the end of the expansion's act, who you would think would be tougher. Even if a boss is too strong, you can always go back to cleared areas to level up further. Enemies don't respawn after you leave a zone, but all areas are repopulated every time you save and quit the game. In a move that I loved, Blizzard doesn't allow you to save the game unless you are quitting. This means you cannot save before a tough battle and reload if you lose. While you never die permanently, each death take a significant amount of gold as a fee and dying too often can put serious strains on your gold reserves. I thought this added a great amount of intensity to each fight without making death too much of a penalty.
The levels are randomly generated, both in layout and in the monsters that populate it. No matter how many times you play the game, you will never be able to know what is coming next. It will take several playthroughs to see all of the enemies that are in the game. One thing worth noting is that each level is not meant to be traveled through straight from entrance to exit like most games. It isn't rare to walk into a new area but find the exit just fifty feet away from the entrance. To have any chance against the later enemies and bosses, every corner of every level must be searched and cleared out. The game never actually explains this, so newcomers may make the mistake of trying to run straight from start to finish. The game is separated into four acts with a fifth being added by the expansion. Each act has its own town that acts as your home base for healing, information gathering, buying, and selling. Every town looks quite different than the others, as do the areas you will explore around each city. There is far more variety in this game than in the original Diablo. Basically, if you want a sense of scale, the entire first Diablo is roughly as large as any single act in Diablo 2 (save for act 4, which is notably shorter than the others).
There are five classes that come with the regular version of the game and two more are added with the expansion. Each class plays drastically differently, so there has to be one that will suit your tastes. While the choice of seven different classes is great, it also hurts the game slightly when you are looking for new equipment since most weapons and armor are class specific. In the first Diablo, there were only three classes. This meant one out of every three items dropped by enemies was something that could be used by your character. Now, the odds have dropped to one in seven. Since it is quite common for an enemy to drop items that are worse that what you already have, there can be long stretches where you don't find any new items. I actually went through an entire act without finding one useful weapon or piece or armor. This is a little disappointing when you consider that the game is driven so much by the loot hunting.
Since you will be so overwhelmed by waves of enemy forces, the game tries to help you out by allowing you to hire a mercenary to fight along side you. Each town has a unique mercenary type for hire and each has their own strengths and weaknesses. These mercenaries are, by far, the weakest aspect of the game. While it is cool to be able to level them up separately and give them your extra equipment that you don't need, their AI is so bad that it's hard to believe that it isn't bugged. Many times, you run into a fight only to find that they got stuck behind a wall and won't follow you until you run back to get them. Other times they will run straight past the fight and pull more enemies on top of you. Sometimes they may just stand still while you do all of the fighting. There were countless times when I died simply because my mercenary didn't want to help out. I know this game came from the early days of ally AI, but there has been plenty of time to patch this problem.
The story is just enough to push you forward. It's a generic tale about the war between Heaven and Hell, but it stays interesting with amazing voice acting and interesting characters. The cutscenes are up to Blizzards normal top notch standards and still look good today. The game has tons of replay value, not only because of its random design, but because of two more difficulties that are unlocked after completion. These difficulties cannot be played by a new character, but instead must be started by continuing on with a character that has already been through the game at least once. The game already takes from twenty five to thirty hours to complete and these added difficulties triple that number.
I have always been a huge fan of the two dimensional, isometric art style employed by Diablo 2 and other games like it. It gives the game a hand-painted feel that helps the outdated graphics still look great today. Even though there is much greater variety, I do think the original Diablo actually looks better. Even still, I never ran into anything that looked bad and hurt the experience.
Diablo 2 definitely is able to compete with any of today's games in the genre. Even with its few problems, the draw to find that new sword or make that next level is too strong to resist. With the Diablo Battlechest (which contains the original Diablo, Diablo 2, and the Lord of Destruction expansion) available at a great price now and Diablo 3 coming soon, now is the perfect time to get caught up on a great series. In my opinion, there still isn't another action RPG that could be considered better than Diablo 2.