I agree with most of this. EQ's "hell levels" are something of a red herring, however, given your point about the game becoming dull. Hell levels were more of an annoying glitch. Everquest didn't become dull because of hell levels, it became dull because most classes didn't have enough to do in typical combat situations.
Big penalties for failure, a low tolerance for minor mistakes, and a limited set of tools for players to win with meant that typical combat scenarios tended to have very little variance. Players usually did the same thing every time. In WoW there's a lot more room to experiment and recover from mistakes without losing the battle. The result is that for most people, battle tends to be more dynamic.
There's also a huge install base in Internet cafes in China and Korea. College kids spend a huge amount of time socializing online there, which is where the major marketing push is made.
Big penalties for failure, a low tolerance for minor mistakes, and a limited set of tools for players to win with meant that typical combat scenarios tended to have very little variance. Players usually did the same thing every time. In WoW there's a lot more room to experiment and recover from mistakes without losing the battle. The result is that for most people, battle tends to be more dynamic.