The retro/classic look! I agree, there is something pleasant about them. Maybe it's the nostalgia, but I'd happily love a few retro keyboards in my collection.
A pen tray is definitely useful, though it isn't a must for me. Doesn't make your preference for a pen tray any less valid, I imagine whatever it is you do, you always need a pen nearby in your workspace xD
I personally don't like my keyboards to be very loud, so I like my cases to help mute the sound a bit without making it sound weird.
I read recently that it is actually healthier and more ergonomic to have a flat keyboard or even a negative typing angle. Our wrists benefit more from it.
Thank you for sharing the pics!!! Those are indeed some very classy looking keyboards