I noticed some people are a bit disappointed that Act Bosses don't give good loot.
But Blizzard have really polished this game and there is a reason behind a lot of their design.
In D2 there would be a constant boss run grinding, the same thing over and over, negating the need to do anything else but boss runs for items.
Act bosses in D3 don't drop good loot, instead provide the function of stopping us from progressing the next Act where better Tier items drop.
The loot that drops in nightmare act 1 is not as good as the loot that drops in nightmare act 4. There are many different Tiers of loot and to get that loot you have to reach certain parts of the game on certain difficulties.
So the point of these bosses is to give you a challenge to earn the right to farm loot from champions/elite monsters in the next act. When you get into Hell mode you will likely find that you need to get new items before you can challenge the act boss and earn the right to the next Act.
Once you're in the next act you can farm away until you feel strong enough to take on the next boss. Eventually earning the right to fight in ACT 4 Inferno will grant you the possibility of finding the best items in game.
I think this system sounds really good.
