If you play a MMO to the extent that many MMO players do, $10 to $15 a month isn't a lot of money for the game; after all, I've known people who played 10 to 40 hours a week on some games for several years.
The disc space issue is another common trait of MMO games. Typically, a MMO game will receive regular (and free) content updates, and these content updates add up over time.
Now, I personally wonder whether the pricing model of MMO games is appropriate because I think that a good MMO game could probably benefit by giving away the game and all content updates (including expansion packs) and just maintain a monthly charge. After all, it lowers the barrier to entry for most consumers and (as long as you provide regular content updates) it prevents users from finding their current level of content boring leading them to quit. The reason I think developers don't choose this route is that there is probably a very high number of people who don't play past the first month, and their total revenue is much higher this way.
