That is just to help wean Diablo players off of action RPGs.
From the chat;
"<Larian> The game is split in 4 acts - one act takes place in a dungeon (the first), the second and third outdoors and the fourth in a big tower"
From other comments, the first act is 5-10% of the game.
"<Arhu> RiftRunner is supposed to be a bit more tactical, and better, of course <img src="/ubbthreads/images/graemlins/winkwink.gif" alt="" />
<Lar> but Riftrunner has a totally new and rebalanced statistics system ?
<Lar> With brand new combat equations
<Lar> ?
<Arhu> Yea
...
<Arhu> As a result, monsters will be stronger and more challenging...
...
<Lar> Referring to the hack & slash question - There will be less monsters and the fights will take longer."
Riftrunner FAQ 
