Real-money trading is one of the most debated aspects of MMO gaming at large -- some games don't actually charge a monthly fee, and instead what they do is sell ingame items for real world money. Want that hot sword for your character? Put in your credit card and pay up. Blizzard, obviously, has never really subscribed to the idea, since a lot of players think it's unfair to make how much money you have in the real world a part of the game you play. Nevertheless, there is a lot of money to be made in selling virtual items for real money, and Blizzard has come up with their own form of RMT in terms of server transfers, name changes, and now gender changes as well.
Blizzard has rules for their RMT, though, and Zarhym lays a few of them out: they won't charge for any item that means anything in game -- cosmetic items and looks are fair game, but actual gear or "integral services" (whatever that means exactly) is a no for them. They won't charge for anything that was free before, so creating up to 10 characters on a realm, for example, will always come with the subscription (though adding more may eventually be possible with an extra charge). And Blizzard's RMT comes as a game mechanic itself -- they choose to charge for things not just because there's a cost for them, but also to "curb their frequency," to keep all players from doing them all the time.
It's an interesting idea, and it's definitely a lot more player-friendly than charging for things like, say, horse armor. You could also argue, of course, that something like the WoW TCG is also a kind of RMT scheme, since you have to pay real money for real cards to get in-game items (even though Blizzard has made sure those items are cosmetic as well). But paying for transfers and changes is a little sneakier -- Blizzard is slowly wading into RMT, so far successfully dodging all the sharks in the water.