The main concern I have is that instead of a small, independent company with the sole purpose of creating the best VR product they can, they are going to be subject to performance targets and alternate goals, like form factor, over quality of experience.
It would not surprise me, for example, that instead of getting the absolute top of the line in terms of display technology, they may now be forced to go with a "good enough for the masses" approach to keep consumer costs and size of the headset down to encourage mass market adoption.
Facebook is very much in the market for mobile applications, and while OR was always planned for mobile as well as PC, it had a very PC centric focus which I could see changing to the other side of the spectrum with the acquisition.
I hope I'm wrong.