I hope, they really pull something out of the awesome bag with them.
I know before they started to overhaul them, that you could customize how/when they do abilities etc, people said it was like Dragon Age companions. (I've not played DA, so can't comment on it)
I don't know if you have seen this video, but it is the best Ranger video out there, even though it is old, they talk about how pets worked before there, so you get an idea of what they want from them.
http://www.youtube.com/watch?v=kAh1OHBI4uM