I also posted under Apple Support: "yosemite font size is too small for vision impaired" and " How can I uninstall Yosemite and go back to Maverickc" etc.
This is not only an issue about retina vs non retina. I also got iMac Retina and came across Yosemite because of this, since I cannot install Mavericks on it. Definitely font readability and contrast is still the issue on imac Retina or any retina machine, since it is due to selection of fonts (type, size, contrast, thickness and rendering that is inherited by the OS X). For the first time I am spending hours to play with the "color" settings in the display on my iMac retina (and tested and had to do this on macbook pro retina on Yosemite). I agree that font blurryness is not there, but this issue is not only due to blurriness that everybody thinks, it is about font rendering. Font type: Helvetica is one problem, narrower,harder to discriminate on eye, thus people are more comfortable on Lucida Grande (wider fonts) on small text, thus easier to read. This is something like Arial was better readable on XP than Calibri on win 7. Calibri would look good on power point for large text but not for smaller desktop fonts. Second, In yosemite Apple chose for the first time "thin" "Light" fonts, meaning stg like, instead of Calibri, it is now Calibri-light, so fonts are now thinner, and lighter, grayer (rather than dark black). Some people blame Jony Ive, don't know who, and I don't care but if you put an esthetic person on a serious productivity issue like fonts and contrast, then this happens. Thinner fonts means harder to read (recently this fashion propagated to some websites fonts, some sites saw the issue and corrected back), and each alphabets sits on less pixels when rendering. Degredation will be worse for non retina displays as expected.
Just check out calendar icon with number 7 on it on Mavericks vs Yosemite when you make the icon small, you will see that thicker and darker fonts on Mavericks will be easier on human eye to discriminate. When you make the fonts thin, it "looks" good from a distance for larger cases but when you try to read and understand smaller fonts, you will feel eye strain more and more. Interestingly Windows font rendering is also thinner fonts after XP and as you know after Steve Jobs, Apple sit back and started implementing changes. They were checking on windows and Samsung, and font issue is one unfortunate decision (this is only my guess).
The last but not the least, awful selection of color/contrast theme: In yosemite there is too much white and gray, rather than gradient and light blue colors. I am not fan of 3D stuff but previous UI was very good in discriminating the main object from the background. Now with Yosemite it is harder. Apple is aware of this, they added increase contrast and reduce transparency to improve on this, but actually this does not improve at all, what we need is to increase the contrast in the front object, meaning that fonts have to be fatter/darker/thicker, background should be light (light blue/light gray). They should add an option similar to iPhone that "bold-font" option. I am using my iMac retina in lower resolution than "best for retina" option, it makes the fonts a bit darker, and looks fatter, with the cost of slight loss of sharpness, since it is so many pixels, still I don't loose the sharpness much. I tested this whether this is related to iMac Monitor problem, and installed Yosemite on my macbook pro retina, so the hardware is the same as Mavericks. I put lucida grande on yosemite, still really saw that readability is worse, since again the fonts are thinner and grayer. People in Apple needs to understands that fonts are not about aesthetics when it comes to readability or productivity. But these days how many people are focusing on productivity?
