Apple spoiled us with displays and have made us expect nothing less than "retina" resolution on all their products. I have owned every generation of the iPad and always thought the displays looked great. I am now addicted to retina. Between my iphone, iPad 3, retina macbook pro, and thunderbolt display (I know it isnt retina, but its extremely sharp), my eyes have gotten accustomed to retina displays.
I can honestly say reading text on the mini gave me a bit of a headache. When I would stop using it my vision seemed fuzzy for a little bit. I LOVED the size, weight, design, absolutely everything about it, except the display. I believe this is due to being used to high res screens on all my other devices. I am 28 years old with 20/20 vision. When I had my ipad 2 and older macbook pro I never got headaches. Always thought the screens looked great.
I believe that the people that are having no issues are the ones that are not used to retina displays. I almost wish I wasn't, because I loved the thing. I don't care much for specs, its about the experience for me. I returned it after two days of use and got an iPad 4. I am still thinking about the mini because I loved everything but the display on it.
There seems to be two sides here where people adamantly defend the mini, or find reasons to bash it. It did not live up to some peoples expectations (mine included, but because apple has led me to believe they were moving towards all retina devices with each new release), but for others its awesome. I wish the display didnt bother me, I really do.