It is! I like it too... BUT the 21.5 is with 70% LESS pixel density and it really looks like a crap after owning a retina display Macbook...
The iMac's display is beautiful, but lacks HiDPI. Apple has spoiled us with Retina Macbook Pro's and iPhones/iPads.
It is certainly a big difference having a Retina screen, but to be honest with you, if you sit 50-60 cm away from a 27" iMac, then the text sharpness is good.
I think we are at least two years away from a Retina iMac. We are talking double the resolution, so the OS has to be optimized for that and the graphics card also...I think the graphics card might be the biggest issue here...