I hear the refresh brings a bunch of new stuff, but this is what I'm most excited about. I wish every website and application had a setting like this:
1920×1080 external monitor? "Comfortable" looks great. 1366×768 laptop LCD? "Cozy" or "Compact" gives you a ton more screen real estate while still keeping text readable.
I used to use Greasemonkey scripts to keep Gmail usable on my tiny Samsung netbook, but it was a hassle and made the site ugly. The new layout settings give you a one-click setting to make the site look great on any size screen.