So far I've read (I'm not expert in optic design), there is a large difference in the optic design of the lens : it's manual, due to a larger (and heavier) group of moving lenses inside the lens itself (sorry for my poor english). A lens having an AF has a smaller (and lighter) moving group, so that the AF engine CAN move it. The result is that the optical quality can be better, with such a manual lens design.
Well, I'm owning one (35mm F2), and I'll should keep it... The IQ difference is very noticeable. Not to mention the colour rendition... That should inspire other manufacturers...
Hmmm, not convinced on that. Whilst it may have some truth for cheap light weight lenses there are some pretty hefty elements out there being moved by AF.