The thinner iMacs are much nicer to carry around, especially if you need to bring them back to Apple for service. The 2009-2011 iMac 27s were beastly and unwieldy. I for one appreciate the weight savings with the thin design.
I think it's supposed to be in that thicker stand base, but for the record, I'm not a fan of that Surface Studio type design. I still would want the internals to be behind the screen.