Yes, as you noted, Johnson and Mies did the Seagrams Building, but Johnson took that modernist aesthetic and ran with it. What might not be known about the Glass House is that there is a connecting, less transparent building in brick that contains the bedroom. Here is Johnson himself in 1949 leaning against that other building. (There are many, many lovely images of that Glass House in the property with tons of green penetrating it. Perhaps it is not as sterile as it might seem.)
It has been my impression that this was a kind of sculptural example of an ideal house and not necessarily where Johnson lived.
I saw a public TV show on his art collection once. It was an amazing collection of quite contemporary work, from Pollock onward, if my memory serves me well.
Re the Seagrams' top floor dining room, The Four Seasons: at one time there was a set of Rothko paintings there. But they were heavily damaged by the light and removed. They looked a bit different from other Rothkos, with a sense of a gate instead of a wall. I can't escape the visual connection to the David Smith Cubi that sold for so much lately.
Finally, re Johnson's AT&T. It does sometimes seem like a huge, blank piece of postmodernism. But there is something pretty overwhelming and powerful nonetheless when one enters the lobby. Maybe it is not so easily dismissed as we might wish. He originally wanted a block or more of buildings torn down in front of the AT&T to create a more effective setting and piazza.
I guess Johnson's real value will shake out over the next few decades. Same with William Rubin.
P.S. Thank you Leni for your so kind comments. You made my day. :)