Disappearing Buttons
I thought you discovered disappearing buttons years ago. The MarsGlobalSurveyor site opens a subsidiary window to view the images, without any button bar.
I personally dislike this, since I like to control things with the buttons - like If I'm browsing a whole bunch of images on MGS.com, I'd like to step back through them sometimes with the "back" and "forward" buttons.
As for The Nexus, I couldn't have described those partial loads as clearly as you did, but I understand the process and can live with it, so don't worry on my sake.