This is really a review of the actual game, but I played with the expansion pack instead of playing with and without it.
Basic Characters:
Necromancer: The Necromancer is master of the dark arts. He lacks personal combat skills, but raises an undead army to do his bidding. He'll attack with spells and uses wands to assist his reanimated minions.
Amazon: The Amazon specializes in the bow and spear. She is an independent character who may be difficult to use at first but as she levels up, becomes increasingly easier. Bows, crossbows, Javelins, and spears are her primary weapons.
Paladin: The noble Paladin fights with the help of devine blessing to purge the land of evil. The paladin uses Auras to aid him in battle while he fights his foes with sword and shield in arms. His Primary weapon is a sword or mace of some sort along with a shield in the other hand. The paladin is extremely useful in group play, but requires a little skill by himself.
Sorceress: Using the power of magic, the Sorceress attacks her foes from afar. Using the three primary elements of the game, fire, lightning, and cold, she stops her foes in their tracks. Recommended for group play.
Barbarian: Possibly one of the easiest of the basic characters for beginners, the Barbarian is extremely strong and has a lot of hit points. However, he generally lacks dexterity and mana. The Barbarian is one of the most versatile characters in the game. He can use almost all melee weapon types and has a special perk; multi weapon weilding. While the other characters can only use one weapon at a time, the barbarian can equip a weapon in each hand. The Barbarian also has war cries which aid allies and himself or affect the enemies. No matter how you build your Barbarian, he's a force to be reckoned with.
Bonus Characters:
Assassin: Using special weapons and martial arts skills, the assassin takes down enemies with ease. However, if he's caught in a large fight, he won't be able to stand his ground. The Assassin also can set traps, in which he weakens or even kills foes. Best used in group play, but possible for solo play.
Druid: A master of the natural world, the Druid summons aid from creatures and nature spirits as well as inflicting natural disasters upon his foes. He also can shape shift into a Werewolf or Werebear for a temporary amount of time. The Druid is recommended for experienced players and does well solo.
Secret Cow Level:
Diablo's hidden secret is the cow level. Only available in the expansion pack, it can be unlocked after defeating Baal. First, return to the First Act, the Rouge Encampment, buy a tome of town portal. Go to Tristam, find Wirt, and take his leg, picking up all his cash is optional, but recommended. Wirt was a character from the last game. He was a merchant, which explains the cash, but the leg you pick up is actually a peg leg, he was crippled. When Tristam was invaded and ransacked, just about everyone escaped. Wirt was hindered by his false leg and was killed, explaining why his body is there. After taking the leg, which is also a somewhat lackluster, but still useable, club, return to the rouge Encampment and put the leg and the tome into your Horodric cube. Combine the two of them and a portal to Moo Moo Farms is opened. There, you will be relentlessly pursued by Hellish Bovines with unrealistic moos. This level is extremely difficult because the bulls are extremely tough and your laughter may make you completely incompetent. If you manage to survive long enough to find him, do not kill the King, you'll never be able to re-enter the farms with that character. As long as the king stays alive, every time you log out and log in, the Bovines will respawn like any other wilderness area.
Multiplayer:
Diablo II has an active multi-player server. Team up and fight hoards of evil or create a battle royal out in a field and see who's the best. However, you must create a new character for multi-player. Also, beware of item theives. If you enter a server, another player may kill you just to loot the body before you retrieve your belongings. Be sure you join the party before you leave town if you want to be safe in the server.
Summary:
Pros: Addictive gameplay, incredible storyline, remarkable re-playability, online servers for public/private use, COW LEVEL
Cons: The Monsters become stronger than you very quickly, so be over-prepared rather than sufficiently prepared for all fights, especially bosses and dungeons, Multi-player theives.
The final Word: Diablo II is an amazing hack and slash game that carried the cult phenomenon torch for years while Blizzard developed Diablo III. For its time, Diablo II was cutting edge graphics, had amazing online play, and was the latest and greatest in RPG gaming. Still today, many find themselves drawn to this game for its incredible story and playability. I would recommend this to anyone who has hours upon hours to "waste".