Some of my thoughts:
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GRUB is mostly just a grey screen to me, I think that'll be fixed up in 16.10.
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I think the login screen looks dated, but I'm fine with the logout/shutdown boxes, and lock screen.
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I agree. There's been no success with bringing the indicator over neither:
4 and 5. I don't use these, so no comment. 
@ouroumov: Some behaviours you describe there are different under Compiz.
2: With Compiz, you can configure the desktop layout to be 1x4 instead of 4x1 (or 2x2 if desired). Not sure if possible with Marco.
3. Compiz shows a nice preview of the desktops when switching.