Mirror lock-up would be my other vote.

Only my K2 and KX have it, some say you can persuade an MX to do it but I've never managed to do that trick reliably (happened once by mistake and I thought it had jammed until a second press took the photo and dropped the mirror again). Bronica only got around to fitting it by the ETRSi, which is even stranger when you consider the size of the mirror.