I don't play MMO's -- don't enjoy paying monthly for a game after I initially purchase it.
When I buy a game, I want to feel like I actually own it, not that I'm renting it (which monthly subscriptions feel like). Look at Valve and TF2 -- they've been rolling out large content updates since the game's launch in late 2007, and they never charged for any of it. Admittedly, TF2 is not even close to the MMO genre, but I think genre is irrelevant. Monthly subscription fees should not exist with any game. Just my two cents.