With the release of patch 1.03 for Diablo 3, Blizzard has announced that new users who buy the game digitally may have to wait up to three days for their game to fully unlock.
The restriction caps players at level 13 and up to the Skeleton King on Act 1 until their payment goes through. Blizzard warns that this can take up to 72 hours. Generally speaking, it'll only take as long as half a day or until your payment clears.
The precaution prevents users with fake or empty credit cards from fraudulently creating accounts to spam users with phishing scams or gold ads.
Additionally, it prevents users from issuing a chargeback after purchasing the game to get the game for free. Valve deals with the matter differently, by locking any accounts that issue a chargeback to the service. Whether their method is better or worse is up to debate.
If you don't yet own a copy of Diablo 3, you can pick up the game from a retail store or order a boxed copy, and you won't have to deal with any delays.
If you're outraged by this development, get mad at the people who abuse the system to acquire free games instead of Blizzard.