HD monitor looks blurry with Retina?
Nothing is going to look as good as a Retina display. If your display seems like it should be sharper make sure you are running that display at it's native resolution. Scaling the display to non-native resolutions can make even the best monitor look pretty bad.

That said I run my MacBook Pro's retina display at it's maximum resolution of 1900 X 1200 because it still looks sharp to my old eyes, even with scaling on.