Google's Daydream VR is, to put it very short and sweet, a big deal. Mobile VR, up until now, has topped out at the Gear VR level. While mobile VR in its current form can be intuitive, useful, and massively fun, it's nothing compared to fixed VR solutions like PlayStation VR and the Oculus Rift. Current mobile VR experiences on the most basic level are just a taste of what VR is like in its current most advanced form. Hooking up your phone to your PC and playing your Steam library in Cardboard via TriniusVR is possible, yes, but it's far from ideal; compared to an Oculus Rift or HTC Vive doing the same job, a phone with Cardboard is bound to be limiting, with wonky head tracking and controls based on whatever's at hand. Daydream is set to solve a few of those issues with content originating from a Daydream-ready phone.
Daydream-ready devices bottom out with a powerful Snapdragon 821 processor, 4GB of RAM, and a number of special sensors made for VR. This means that Daydream already has a few huge advantages over Cardboard, namely smoother tracking, better control, and more technical horsepower. Perhaps the biggest difference, however, is the controller; the controller that comes with Google's first Daydream headset is a motion controller with a small amount of buttons and a clicky touchpad, not unlike the Oculus Touch controllers or the Steam Controller. That specialized controller allows for a wealth of more interactive content than Cardboard could allow by default.
While we have yet to see how much better Daydream could perform than Cardboard when hooked up to a PC via an app like Trinius or NVIDIA Gamestream, but based on the minimum requirements, a few basic guesses can be made. Thanks to the array of sensors required for Daydream, head tracking will be better. Thanks to the more powerful processor, gameplay will be smoother. Thanks to the screen requirements, things over the stream will look good. As for media playback, Daydream will likely be more immersive in general by virtue of its headsets, though passive media experiences can only be enhanced so much by VR. The first Daydream headset is made of breathable cloth, and the phone snaps securely in place. There are no gaps where light can seep into a user's field of vision. While there are some truly wonderful Cardboard-compliant headsets out there, the point of entry for Daydream is higher, which mirrors the general difference between them in all applications.