I thought this quote was telling:
In terms of gameplay and mechanincs WoW is the equivalent of a cheeseburger prepared by a gourmet chef: Nothing original, but the contents and preparation are first rate. What makes the difference is that while World of Warcraft is a gourmet burger, you can get it for the same price as all those greasy McBurgers that have been sitting under a heatlamp for six hours…
It’s not that WoW is anything spectacular and original – it’s that it’s extremely well made and that it has no serious competition in terms of quality.
The Daily Grind: What does WoW do better than any other MMO? – Massively