Please, please, please give us a back arrow in Lightroom. I am often working on an image when I need to check something in another image. Getting back to the image I was working on is time consuming and frustrating.
A back would be great for easily comparing two images on the fly, without changing ratings or making collections. And please extend that concept to include a Recent list so I can quickly find images I have recently been editing. The recent list seems wasted on Catalogs.
I've used CTRL-Z for that as long as I haven't made changes to that second image or taken so many steps that it would be impractical to CTRL-Z all the way back. I don't know if that method works between folders, though.