As a single player game, Diablo 2 is pretty boring, which is an odd thing to admit, because I’ve spent more time with D2 multiplayer in the last few months (first with the QT3 Battle.net group and now with Jon and Sven with the Nezeramontias mod) than any other game I have ever played.
Normal mode is a cakewalk. It will never force you to make interesting character focus decisions - you can pretty much just spray skill points wherever you like and one or two points in any damage dealing skill will wipe out most enemies. Laughable gear is godly in Normal mode. It is just incredibly simple that gives Diablo 2 the illusion of really being a boring game, because immune monsters and elemental resistance penalties are not yet in effect and have not yet stressed to the breaking point your character choices or your equipment.
The real game is in Nightmare and Hell mode. Normal is an extended tutorial for these, but because of the way people play single player games, they never really try to continue into nightmare once they’ve beaten Baal. I certainly felt, in single player, that Nightmare would just be playing the same game again, which is just completely untrue.
Playing in Nightmare single player is untenable for the beginning player. It assumes a focused character design and some great equipment. The first is plausible (although you probably need to be really familiar with the game in multiplayer to make a good solo character for nightmare and hell) but the way drops work in Diablo 2 basically precludes the latter. Essentially, most characters on Battle.net get their killer equipment by playing in high volume player 8 games where the rate of rare and unique drops are exponentially increased. They also get cast off equipment from higher level friends. You can play in players 8 games single player and get this sort of equipment, but I don’t think there are many advanced players of D2 working over the single player game anymore, and your character would have to be godly to be able to handle Nightmare or Hell mode in single player with players set to 8. As an aside, the lack of decent drops in single player games robs Diablo 2 of a lot of its appeal: the thrill of an amazing item dropping from a monster that will make your character a lot less vulnerable is probably the most charming aspect of D2. Let aside the fact that no single player will ever complete a set or a rune word.
Nightmare and Hell modes really show Diablo 2 to be a team game at heart: with the amount of monster running around with resistances and the immense damage being dealt all over the place, every character (with the exception of a tri-focused, heavily twinked sorceress) is going to run up against entire packs of monsters that they are essentially powerless against. There aren’t enough skill points to make it plausible to deal with all possible combination of immunities. You need team members to make sure you’re not constantly running up against a wall, unable to advance. Also, considering the advantages in loot and xp of playing in high player volume games, it is pretty clear that Blizzard meant Diablo 2 to be mainly an online cooperative game.
All of this combines to make Diablo 2 just an immense game (even more incredible with the 1.10 patch), but I can see why people who have only played it single player would consider it to be boring. They are never seeing the core game and they are never being challenged. And then if they do decide to try Nightmare they end up getting obliterated almost immediately. All I can say to the “Diablo 2 is boring” crowd is that they should really do a multiplayer session sometime - D2 is at its worst as a single player game, but maybe the ultimate game as soon as you get yourself a decent group together.