Now, I know what you're thinking. How many of those trial accounts are scammers, so of course they don't make it to level 10? Good point. I don't know the numbers, though I'm sure Blizzard could just look at how many of those accounts made character names by facerolling. However, the account thieves aren't strapped for cash and have been known to not only purchase full accounts to do their evil bidding; but will also reactivate lapsed accounts, slap an Authenticator on and farm/scam away. So the 70% may very well include a large percentage of potential subscribers by process of elimination, since the scammers can afford multiple accounts.
As I've said before, the pre-level 10 changes we already have help us introduce our loved ones to the game. And I think we're all looking forward to the new starting zones and leveling experiences we'll find in Cataclysm. Particularly now that we've had a taste of the new quest mechanics in the Love is in the Air quests, such as PIlfering Perfume and Hot on the Trail.