When I say mobility, I'm referring to mobility within the game. Within a non-VR shooter I can, in one continuous motion, run into and knockdown a guard, shoot his friend, jump through a window, run across a room, leap over a crate, parkour up a wall, grab a rifle, and shoot another flanked enemy.
VR gameplay isn't remotely comparable. Perhaps it's due to VR linking us with our real-life limitations. In real-life, I doubt many untrained people can turn 90 or 180 degrees, raise a rifle, sight in, and fire accurately in a under a second.
I'm not saying there aren't great VR games but VR doesn't appear to add new elements to the genres or gameplay beyond "now try this without an accurate mouse/keyboard/controller" or "you can now see a virtual representation of your hands... and use them to awkwardly manipulate the environment".
When you say "Blaston.. (provides).. gameplay that would not be possible in another medium", I think that's what you're referring to.. the awkward manipulation of the environment. Blaston is a very simple 3D game that absolutely could be played without VR but wouldn't be nearly as goofy and fun. Perhaps that's it, perhaps the "akwardly manipulating/traversing the environment" is the unique gameplay that VR adds (similar to Kinect). The human element I guess you could say.
It seems as though the trade-off is a boost in immersion for a simultaneous reduction in game and gameplay complexity.