I have to admit I'm stunned about Rift Mage. I had previously been a little bewildered by its description at the website, which always struck me as vague in comparison to the images we get of Knight Enchanter and Necromancer.
Right now I feel as though the Rift Mage spec might not have had as strong of a concept and ended up being a collection of spells that didn't quite fit in the base trees. The other specs probably have some of this too, such as with Walking Bomb or Haste or what have you. But those have non-elemental flavor and don't feel lifted directly from the primary elemental trees. The trees were obviously designed well after some spell choices had been made and coded (Vivienne casts both Stonefist and Firestorm in earlier demos). Having a less clearly defined spec might have been a contingency plan in case tree design got difficult.
Firestorm's apparent relocation to Rift Mage makes me wonder if there had once been an Earth tree concept that included Stonefist, but it was aborted (because reasons).
On the one hand: as someone who enjoys specializing in an element or pair of elements, I am excited by the option to again choose a combination of my liking. It made no sense in DA2 that in order to perfect my cold spells I had to take two flashy fire spells as well. On the other hand, I regret the loss of non-elemental flavor, which made it possible to build fire mages that felt completely different. A fire mage who is also a healer, or a fire mage who also uses life drain and death magic, or a fire mage who uses glyphs to control the battlefield—all these suggest different characters for role play.
Given the apparent Four Tree Restriction, I wish we'd had four trees divided between elemental and non-elemental spells, e.g. Earth spells on one side, Spirit on the other. That way you could avoid Earth spells if you wanted to combine Lightning and Spirit, for instance.