If it wasn't already obvious, Blizzard put together a page on their official website making clear their stance towards buying in-game gold, and have just recently given it another big push. To put it simply: don't. The page outlines what we at WoW.com have known for quite some time (hence our collective stance against buying gold) -- that gold buying harms other players. The site doesn't go into specifics other than to say that gold selling companies often acquire their gold through unscrupulous means.
They sum up their statement by saying that "players who buy gold are supporting spamming, botting, and keylogging." Basically, if you're a gold buyer, you're part of the problem. No, seriously. Gold sellers acquire gold by hacking into other players' accounts, taking their gold, selling all their items, and sometimes maliciously deleting their characters. That gold you think some Asian spent hours farming in Nagrand or something is more likely to be some other player's hard-earned gold and the seller is just as likely to be some dude from Jersey.
As tempting as buying gold may seem -- and I've read many arguments towards why people buy them -- the bottom line is that it is harmful to the game and you're not doing yourself any favors in the long run. Blizzard says that it "diminish(es) the gameplay experience," but that's putting it nicely. Gold selling and power leveling are against the EULA, anyway, so anybody who patronizes these services are in danger of getting banned. And if you don't believe in buying gold (go you!), protect yourself by getting an authenticator or reading up on account security.