A lot of people may have already purchased a copy of Diablo 3 digitally and got playing straight away (well as soon as its downloaded,) but now that will not be possible as with the 1.0.3 patch new users who have bought the game through the Blizzard online store will have to play on Starter edition for anywhere up to 72 hours:

“Digital purchases do require a review period before they kick over from Starter to Full editions. We apologise for the inconvenience, but it is a necessary step to combat fraud and other malicious activities that can weaken everyone’s play experience, The delay is no longer than three days, and is often much quicker than that. Hang tight.  ”

If you have played the beta for this game then you may already know what this means, for those who haven’t it will only allow the player to do certain things within the game including:

1. Only Act I up to the Skeleton King available
2. Level 13 cap
3. Matchmaking available only with other Starter Edition players
4. No Auction House access
5. Global Play not available
6. Players attempting to connect to Diablo III Starter Edition in a region other than their Battle.net account’s home region will receive Error 12

Source Diablo Forums