beta is super short, about a hour long if you rush from first quest to the last. It ends with you killing a "boss" which is the Skeleton King.
If you just do the last quest, killing the skelly king, it should take about 30 min since the map is not random like how the final game will be.
Demon Hunter is a mix of amazon/assasin
Wizard is OP
Monk is kinda like a pally without auras since you can heal allys and have melee aoe type attacks
Havnt played barb yet but im sure its not that much different.
Witch doctor is like a necro and the weakest out of all classes IMO.
O, also you cant assign skill points anymore, i liked it better when you could have instead of pre-set lvl up skill point distribution.