Give it a few years and affordable PCs will be able to power VR just fine. A device like the Rift will also become much cheaper over the next few years. Given how easy the Rift is to set up (pretty much just plug it in along with a single webcam that sits on your desk), I don't see any other major impediments to it going mainstream once the prices fall.

Mobile VR still has an important unsolved problem too (absolute positional tracking throughout a room). Google and Oculus's John Carmack are working on this problem (independently of one another), and hopefully we'll see some of their work make it into commercial products in the next few years. But for now it's a real limitation of mobile VR devices.