Today was the MediaWiki.ui hack day, and we briefly discussed the Vector typography refresh that's in BetaFeatures right now.
Several people expressed frustration with the choice to make the sidebar and personal toolbar links smaller size, as well as the grey text used throughout. Our text size already pretty small, and frankly making links greyed out like that violates basic Web usability.
The sidebar color and sizing is a major change that keeps me from opting in to the beta on desktop, despite the fact that I have personally used and enjoyed a very similar font stack across wikis for months. Editors use these two toolbar areas many many times a day when editing, and we're making their life harder with this change.
Despite the above objections, I agree with the reasoning behind the change, which is to put content first in order of priority. Even if we use the toolbars all the time, content comes first in the encyclopedia. So I'd like to propose an alternative solution to this: bumping up the content text size to 1.1em, and leaving the rest at the smaller .8 em. On Chrome and Firefox, OSX this computes to a page content size of 14px.
I made quick gallery of what this looks like on a stub, the Main Page, Portal:Current events, a long article, and a Talk page. It's at http://imgur.com/a/yXVTo
If you play around with this yourself, be sure to apply the sizing only to mw-content-text only, not the body, or else everything will blow up in size. ;)