It's interesting they're considering implementing a way for apps to signal to the window manager the size it prefers for its windows. This has been a concept on OS X since 10.0, though it's only ever been used by the OS figuring out what size to zoom to/from when the user clicks the green zoom button. If this feature makes the cut it I'll be curious to see what other uses they find for it.
One concept I'd like to see return in modern desktop environments are 2D grid virtual desktops. OS X 10.5/10.6 had what I'd consider the best implementation of the idea and I loved it. It leveraged spatial memory much better than the linear layouts popular these days, especially with short smooth animations to make movements between desktops more concrete mentally. 2D grid virtual desktops can still be found in more "old school" type DEs like XFCE but the level of polish isn't comparable.