If WoW goes F2P that's it - the P2P MMO market dies that day. Even if it doesn't yet, there's so much choice in the F2P market that new MMOs hoping to base their revenue on subs are going to have a hard time selling that to a substantial playerbase. There's just so much more competition now.
3 or 4 years ago, F2P meant the ubiquitous Korean grindfest. Now we've got Dungeons and Dragons Online, Lord of the Rings Online, Champions Online, Age of Conan, City of Heroes (soon), Fallen Earth (soon)... That's a sizeable chunk of the western MMO market.
Have to wonder what EA is making of all this. I seem to recall that a few months ago they were talking about TOR keeping 100K+ subscribers for ten years. The way things are going at the moment I don't think subscription based MMOs will exist in five years, let alone ten.
Time to rewrite that business plan I think...