a) manual focus & aperture. "Focus by Wire" and not being able to manually set the aperture are crippling the lens.
b) Buy in the longest continuing running lens mount possible. Hence my penchant for Nikon over Canon, and my slow shift to M-Mount.
I do break those rules on occasion - sometimes you just need autofocus in a native mount - but I wouldn't buy into an entire system predicated on doing so.
I wholeheartedly agree with you on point (a). This is quite true with Sigma SA mount lenses at least. You've got me started on my favorite rant. Yes we are basically stuck.
Maybe it is too hard (or impossible) to set up sd Quattro firmware to support full time manual aperture on SA mount lenses, but I heard that the Nikon D850 can do full time manual aperture. Otherwise with the sd Quattro we are stuck with the crippling situation in which the lens closes down to the shooting aperture only at the moment of shutter release.
Focus by wire tech might not offer enough precision at this point, and we might not be able to force "no change to focus setting while shooting" if we are using an electronic lens. That is, the lens might be programmed to change focus slightly at certain distances regardless of whether it is autofocused or focused by wire. Again, crippled! Sure, with Global Vision lenses we can change that programming but then we might end up with a lens that works with manual focus but performs poorly when autofocused. Frustrating.
Edit: and the irony here is that these are state-of-the-art lenses capable of matching the sensor resolution of the sd Quattro series!
Unfortunately there are not many options for real full-manual control with sd Quattro and other Sigma interchangeable lens cameras. Basically we have M42 (most lenses are 40+ years old), or we need to get skilled and do mechanical modifications (mount replacement or worse) to other manual lenses from other manufacturers such as Samyang (Sigma doesn't offer manual SA mount lenses).
On the other hand with the short-flange Sony and Fuji cameras fully manual lens options are plentiful. Too bad none of these have a Foveon sensor!