Nice writeup. I have to agree on the size, weight, and balance. Coming from their 7" development tablet, it's not much of a step down, in screen size. I had to marvel at what they could fit in a $500 device, actually.
I would point out that the depth sensor aids in tracking & area learning more than you probably think. Neither of those functions have been demonstrated to work as robustly on a standard smart phone.
I also wanted to point out that it's standard for SLAM systems to use the IMU (compass, accelerometer, and gyroscope) for coarse tracking, and then refine the estimate by registering with features seen by the camera(s).
Anyhow, I'd be interested in knowing whether your tracking stability could be improved by hanging some posters and putting down some other landmarks in your dining room. While it might like clean, flat surfaces, it doesn't actually want or need them to be featureless. The old tablet actually had a fair amount of difficulty tracking in an empty, white room.
IMO, they need to fund some game devs to make more AR games, or to utilize Tango features in existing hit games. Ultimately, it might be Facebook & Microsoft that make AR happen. It feels like Google still isn't throwing a huge amount of weight behind this.