How did they get this game so right?

[Deathrydar]

I think it comes down to game mechanics and the type of player the game is made for:

If you create a game where the end of the game is the destination, where every class can solo, and where the unique roles of each class blur, you will attract that type of player base.

If you create a game where the journey is the destination, where every class pretty much shines while grouping, and where every class has a particular role, you will attract that type of player base.