Blizzard is adding a real-money auction house to Diablo III. MMO Melting Pot has a roundup of several comments on it.
If the experience goes well, I would imagine that Blizzard eventually makes the same change in Warcraft. Why not?
One difference that should be good is that gold selling will no longer be the massive black market that it currently is. If gold selling becomes legal, like it now is in Diablo III, then gold spam should mostly disappear. What's the point of spamming someone when you can already sell it through a proper channel? Eliminating black markets is generally a good thing.
What I hope does not happen is that it starts to require real money to do be reasonably raid-ready, or to otherwise be in a position to have fun in the game. The monthly fee is enough. I hope the game does not get to the point that unless you are paying a lot of extra money, you can only do a little bit of the starter content.
How this works out for Diablo III should be very interesting. I'm guessing it will be fine, and that we'll all wonder what the fuss was about.