Now here's a tale about the MMO industry and what WoW achieved.
World of WarCraft changed the genre in three aspects: it added Diablo-esque item collecting, never-before-seen user-friendliness and actual option to play solo if wished.
I have played many MMOs till this day, including Ultima Online, EQ2, Matrix Online, City of Heroes, Lord of the Rings Online, Star Wars Galaxies, A Tale In The Desert and of course WoW. And I believe I am finally able to define what made WoW the king of MMOs. One word: connection.
Connection, of course, in various meanings. First: guilds. In EQ2, there were guild houses, in Guild Wars there are guild wars (hence the name), in A Tale In The Desert you could houses and cities with your guild, in Star Wars Galaxies you could (before SOE did some "changes") run a government. But in WoW, you actually had to connect with your guild mates, because of a never before seen raid system: raiding with 40 people. Yeah sure you could make raids in EQ2, but they really didn't need any connection did they? No more than needing a connection for entering a dungeon, but coordination in a group of 40 people? That means you must understand what your GM means when he says "Gah". He means "Warlocks, more DoTs.".
Second: Quests and zones connection. In LotRO, which I am playing, there is no information or connection quests about other settlements. For example, you are level 28, you are wandering in North Downs, do you know how you can find out that there are tens of solo quests that will get you to 31 quickly in Othrikar? By asking someone where it is. Not good. See, I couldn't think of asking anybody where the new quest guys are, because it sounded stupid to me ("Excuse me, I want to know where I can get quests?"). I found it by accident two days later, two days of boredom and total two hours of playing. After finding Othrikar? It took me two days to level up to 31 from 28. In WoW however, you were led to new places by quests. They pointed out the exact location of the new settlement which was filled with quests of course.
Third: The Connection that the player forms with the game. I'm sorry, but the most important thing here is graphics, but not in the orthodox way. Take EQ2 for example. It has probably the best graphics a MMO can have, but does it pull you in the world? No. It makes you feel like you are playing a film, not a video game. You have to make the graphics just right, not too creepy, not too bright, not too cinematic. Not too good or not too bad, just right. WoW did that .
And another thing about the third connection the most important thing: items. Let's admit it, we are greedy. We want tier 531 squared raid sets, we want PvP rank award sets, we want it all. Blizz showed that it can tempt us that way in Diablo, and they tempted us once again in WoW. We stayed up all night making MC raids, ever stopped to think why? Why am I spending my night with 40 people I don't know? Because I want a friggin epic, that's why. I want to show the real members of this fictional world that I have succeeded, I rule and I am number one. I want to beat them in PvP because of my buffs. That was what WoW did (I heard GW was pretty succesful in this area but I've never played it).
But what WoW failed is the urge to please the players too much. They screwed the lore over and over again to make changes that will make the players happy. Dreanei Shamans were one thing, but Blood Elf Paladins? That's when we need to stop.
Overally, there is only one more thing left to say: If you have never played any MMO, play WoW. If you have played many MMOs but now you want to spend some time playing only casually, play WoW. If you want to play a MMO with your friends, play WoW. But if you are a mature man/woman who wants some serious game, NEVER play WoW.