The beta is just the first 25% of Act I.
So i just played the D3 beta today and it got over in like 2 hours ): - i thought it will be longer since it is the complete ACT 1 and the final game only has 4 acts. This makes me think that the final game will not be long at all .... its value will be in replaying the campaign over and over trying to farm items....
Remember how long the D2 demo was? It ended once you killed Shadowraven, it didn't let you play the full Act I. The D3 beta is more or less the same length from what I've read.
You will be missing a significant amount of backstory depending on how well they cover their tracks. Specially because a lot of major characters from D1 whom we assumed dead aren't dead at all and poised to make a return.
Can a total D1 and D2 noob jump into D3?
But the story really isn't the strongest point of Diablo as there are other much better games out there better suited for a proper singleplayer RPG experience.
Diablo is first and foremost about the item hunt. Just how powerful a character you can create and how it fares in Hell difficulty.
Edited by Saumil, 07 April 2012 - 06:52 PM.