Overall the update is awesome. I really like the new fonts and the really like the updated look to the Markdown input box for question asking and answering.
I'm noticing, on my Thunderbolt display (which is attached to my 15" rMPB), that the top bar text has a bad case of the jaggies.
(Direct link: http://i.stack.imgur.com/zPc7z.png)
The Thunderbolt display is running at native 2560 x 1440 resolution.
For comparison here's how it looks on the retina display on the MacBook Pro (which is great):
(Direct link: http://i.stack.imgur.com/XywMV.png)
I'm using Chrome 41.0.2272.76 (64-bit) on OS X 10.2.2 to view the site but it renders equally jaggy in Safari.
Edit: those scaled-down images don't show the problem. Use the direct links to see them at full resolution.