I'd say the GearVR is the more production-ready version of Cardboard, but requires a specific phone that only a segment of customers have. I own a GearVR and S7, and Daydream seems very similar to that hardware while addressing a few of the out-of-the-box issues that GearVR has:

1. GearVR has no controller. Daydream's controller provides both direct and motion input.

2. GearVR has decent resolution with the S7. Phone resolutions should only improve over time, and so will the experience (particularly the screen-door effect).

3. The GearVR hardware is hard plastic and a little uncomfortable after 30-45 minutes. This headset seems to go to some length to avoid that problem.

4. From what I understand, the GearVR has high-resolution sensors inside of it to offload some of that processing from the phone. This has the effect of tightly coupling the GearVR hardware to Samsung, since they're the only ones making compatible hardware (to my knowledge). Daydream is more of a reference, so there will soon be more compatible phones out there.

5. Publishing apps to GearVR requires going through Oculus (Facebook), period. On the S7, plugging into the GearVR is actually a hard shortcut to Oculus. You may not agree that Google is better, but I doubt it's going to be as walled-in.