No veiling flare and good separation of tones in shadows and highlights.
edit - If you are using "sharpness" as a criterion, you are considering contrast whether you know it or not. Sharpness and contrast are inextricably linked.
OK. I recall when I got the Pentax 67 lens that my only criterion was to "get some decent lenses" (I was a bit more specific starting out with the Canon kit many moons ago). Of course now I realise I got much, much more than I bargained for (I almost came home with a Hasselblad kit...) and I'm not complaining, certainly not about sharpness. It's good to have this contrasty lens subject out in the open, as too often a "contrasty lens" is judged as the only one somebody should buy without due and careful consideration to other factors (weight, ease of use, prime vs zoom, others...).
.::Gary Rowan Higgins
A comfort zone is a wonderful place. But nothing ever grows there.