WoW is getting to be on the expensive side, though. Plenty of MMOs out there are from time to time offering packages that are effectively $10 per month (usually as $30 for 3 months or $60 for 6 months). There are also F2P (and Hybrid) models to consider.

The basic cost of the game is also an issue, specially for new players. Last time I looked, unless the pretense player caught a sale, the full WoW with the expansions needed to reach lv 85 costs $120, with only one month of "free" subscription included (40 days if the player picks a trial and upgrades to a full game); it's specially expensive if you think that, although Outlands and Nortrend content is still out there, new players will probably not enjoy it due to the difficulty of finding players at the appropriate level (coming back after outleveling the content might be interesting, but most of the fun is absent due to the resulting ease of the content). Most other P2P MMO games are much cheaper to get the full package, even the ones with expansions

And, although the level of polish in WoW is about unmatched, polish is not the only factor in deciding the best game to play (and, thus, the one that could charge more from a given player). For example, in my case MMOs that offers gear appearance customization, solo paths to earn end-game gear, more flexibility in adjusting characters to the challenge at hand (usually as more specs or respec anywhere options), more flexibility in the number of players for group content, group content that fits in 30-minutes gaming windows, less travel time, or less locks on content (locking gear acquisition instead of locking raids) get extra points when I'm deciding what to play (which is to say, WoW, specially after Cataclysm, would need to compensate with a really higher quality to be my game of choice, and the quality difference is not high enough to justify me keeping my WoW subscription active right now).