I have been thinking about what makes World of Warcraft (WoW) the unmistakenly most popular payed MMORPG:
WoW is one of the most polished MMORPGs there is on the market. While competitors come with better grafics and gameplay they are usually filled with bugs. Not only is there the bugs, there is also major class imbalances, lack of content, uneven spread of content etc. It is simply too hard to compete with the 5 years of constant testing and patching that WoW has gone through.
WoW is easy to learn yet (arguably) hard to master. After testing quite a few MMO:s such as EvE, Mortal Online, Aion, Rift, WAR and more I have to say that WoW was without doubt the easiest to learn. Especielly after the newest expansion Cataclysm where it got even more polished and easy to learn; I started to play in the early versions of WoW. Some games (read EVE and Mortal Online) are so hard to start with that you practically need a friend over your shoulder during the first hours of play and you need several week without help to actually become something more than an ant in the universe.
WoW has good mainstream PvP, good high end bracketed PvP, mass PvP, small scale PvP, large scale raids, small scale dungeons, roleplaying or roleplay servers, a well working economy, solo play, storylines, large open landscapes... The list goes on. It doesn't excel at any of these. No, far from. But it has them all and that is usually enough for most players.
4) No grind-fest
WoW is compared to many MMOs, especially asian ones, not very grind based. Many games require you to do the same work for hours and hours to get anywhere. While WoW used to have this problem early the expansions have more and more filled this hole.
"What the hell!?" - "World of Warcraft have a 6 year old engine". Yes, WoW is not the most stunning game grafically there is, even if many upgrades has been made. The grafical style in WoW is something close to timeless and honestly I prefer it much over many of the competitors. Also WoW doesn't need much computer powers because of this. I have been able to run it well on my 1.5Ghz Celeron laptop and it still didn't look "ugly". Many other games only look stunning with maxed settnings, 4x AA, 16x AF and others have a low polygon count to make up for high res textures making the game technically more advanced than WoW but less pleasing to look at.