Perhaps not as much as we’ve been led to believe: Lens Sharpness.
Ken Rockwell points out that in most real world scenarios, lens sharpness is just fine. Today we pixel peep to the absurd dimension, finding limitations that may not matter. And once we find the limits, there are often simple work arounds – such as stopping down the lens by a stop.
He also notes that sharpness tests are too often done by photographing a flat test chart, which bears little resemblance to our 3D world where everything is not always in perfect focus. There is a lot of interesting info in the above linked article.