I think the PTR was fun to just try out some of the new skills, but other than that, I don't see why anyone would stay on just D3 alone and not get the expansion. RoS unlocks all the fun stuff that everyone wants.
I'm not so sure about the new DH ability...it seems like it would make for a must-use skill for every DH, which is what I think they want to avoid in the expansion, right? It does seem like something they are going to give a long cool-down to though. I just can't wait for that black hole skill on my wizard! OMG...that is awesome.
Ya, I think what's a win here for Blizzard is all of these characters have rich backstories. And this probably makes them feel more identifiable than what you'll see in DOTA or LoL. I definitely think they stand a shot at becoming the top competitor of this genre.