ive played several of the most popular mmos
wow is popular i assume for 1 reason.
ITS SOOOOOOO EASY!
solo the entire game, max levels in weeks, hack and slash your way to the top in no time to....play that "massive?" endgame content lol
It's easy if all you plan on doing is hitting the max level, but there is much more you can do. If you do many of the quests offered to you, it won't be so fast.
There is NO way you can solo it. Maybe by leveling you can, but you're forgetting the raid dungeons. If you think you can solo an endgame instance, buy the game, get fraps (if you don't already have it), and show us.
Most other games make it really hard for you to level, with a high level cap, offer you no quests, and don't give you anything good to look forward to endgame.