Okay, I have to talk about the cinematography of this scene a little bit. It gives me so many feels.
Look at how clean and orderly this room is. Everything is in place, everything has a place, and nothing’s out of order. It actually reminds me a lot of what John’s bunk or tent might have looked like in the military. It’s rigid and unflexible and completely the opposite of 221B.
Now look at the colors. There’s a lot of white relieved by thick chunks of red. It’s clinical, cold, and impersonal. The couch is the only warmth in the room with a chocolate brown coloring. The armchair John’s sitting in is black leather, which isn’t warm at all. It’s like the white, cold and sterile.
There’s no life in this room, no joy, no wonder at what new thing might be coming around the corner. You can look around this room and see everything in maybe a minute or so. Completely unlike the warm and friendly clutter in 221B. That flat, you could spend days looking and discovering something new and interesting about either Sherlock or John and the life they’d taken to together. While John might think he’s moving on, even with finding Mary which is cool, he’s stuck back in a cold and sterile environment. Right now, the light is gone from his life and he’s just staying not living.