Exposure override in small cameras. Being a "simple" camera doesn't mean being a "stupid" camera.
The only real drawback of my beloved Yashica T3 is lack of exposure compensation of any kind (not even a stupid +1.5 EV button).
Lack of mirror lock-up on certain cameras, like the Minolta X-700, which would certainly deserve it.
Lack of a spare battery compartment on the shoulder strap. That should be mandatory on every electronic camera. It's like omitting the fuel reserve indicator in a car. There should be a law about it.
Lack of an ocular shutter on the shoulder strap (again extremely cheap and very useful).
Lack of use of proper light traps instead of felt gaskets. Felt will fail sooner or later. Light traps never fail and never require maintenance.
"Quick-load" mechanisms. They are fast, reliable, and make safer to load the camera while standing. With the traditional method I personally find that I have to sit somewhere and put the camera on my thighs as I feel I might drop it by mistake while performing the film load.