FWIW, I use a late 2013 rMBP 13", and I plug a secondary display into the HDMI port - Yosemite looks far worse than Mavericks did on the secondary display. Fonts are the main culprit - not clear at all, to the point where I find it quite tiring to use the secondary display and avoid it unless I have to.
Fonts are horrible. menubar icon darkness is inconsistent, and badges are unreadable at larger dock sizes than the old badges. Other than that it all looks fine. Oh, and dark mode is hilariously bad on non-retina
I've noticed the same thing. Dark Mode only looks good on the Retina screen. It looks awful on my MBA, my 27" iMac and an external 1920x1080 monitor. The menu fonts in Dark Mode are nasty looking on anything but a retina display IMHO.