I have yet to play an MMO that doesn't feel like an updated version of Diablo 2 with 20% more grind. Playing Battletech with the Rogue Tech mod is making my computer really strain but is definately enjoyable. Makes the game a little more true to the tabletop rules.
What makes a MMO different is the relationship you have with other players. If you don't find decent friends and/or a decent guild, then it's just a more expensive version of an RPG with multiplayer elements tacked on (such as Diablo 2).
Agreed. When you really enjoy the people you play with it makes all the difference. Thankfully I haven't been sucked back into another MMO since Star Wars Galaxies ended.
The originators of MMORPGs wanted to call this type of game "persistent world gaming."
Instead the term that stuck was Massively Multiplayer Online Roleplaying Game, which is even more of a mouthful.
I always thought it should be called social games, but that term is now associate with a particular category of casual games.
The focus really is on other people. Most of what is good or bad about game design in MMORPGs is supposed to change how you interact with other players. The end game content forces you to join guilds capable of organizing raids. Certain XP bonuses encourage you to find a guild while leveling up.
A large number of chat functions facilitate communications between guild members and whatever extra-guild friends you make in the game.
If there are a lot of people doing quests in the same area as you, you'll find that most of the monsters are dead before you can get to them. As annoying as that is, you are forced to either group up with other people trying to achieve the same goals, or else go find some place where other players aren't. Either way, other people are affecting your game experience.
Nearly anything you do in a properly designed MMORPG has to do with other people for good or for ill. It's all about those other people, so the people you play with necessarily have a large impact on your experience of the game. One consequence of this is that different games have their own "culture" among the player base. For example in World of Warcraft, if something goes wrong in group play, people reflexively blame the damage-dealers, but in Warhammer Online: Age of Reckoning, people reflexively blamed healers. The blame thing is annoying no matter who is getting shafted by assumption, but the Warhammer community annoyed me much, much more because no one wanted to play healers because everyone heaped abuse on healers. Even though the game and the mechanics of the game were (in my opinion) objectively more fun than most other MMORPGs, I had to leave because the player community really torqued my 'nards.
Star Trek Online on the other hand, attracts a lot of players who normally don't play MMORPGs (Star Trek fans who aren't necessarily MMORPG players or even gamers). Consequently, the culture of that game is surprisingly tolerant of newbies. Most people will patiently stop and explain things to clueless people when asked.