Awesome gesture from Blizzard, that. Way to show some loyalty to your fans.
Can't wait until someone turns this into a reason to hate them anyway.
As for the "shortness", it's five hours when playing through it specifically to get through it as quickly as possible, not in any sort of regular sense of playing through it. You can get through a lot of games a lot quicker than you'd think if you push hard at it, rather than stopping to fiddle with your character's skills, comparing this item with that, looking through the crafting options, discovering side areas and random special events, as you would be when playing it properly.
If you want to ruin the experience, go ahead and try to finish it in five hours as well. But then you have Nightmare, which will also take about another five hours. Then Hell, for another five hours.
And then because you've rushed through the game Inferno will take you out the back and beat the **** out of you.
Five hours until it unlocks!