The Olympus OM's, especially the OM4 Ti. They are theoretically a great idea and I lusted after them in the days I lugged around my Nikon F2's for work. So a few years ago, and no longer a working photographer, decided to get one and indulge in my dream. But ages pass and now I am a slow film, tripod mounted sort of guy, and getting to the itsy bitsy 'B' setting button drove me nuts, it's almost impossible using a QR plate on a tripod, especially if I had to take my gloves off. And the impossible to turn shutter speed ring when using gloves! So that gathered dust before being sold.
Hasselblad 500CM, my first medium format camera, I hated it. Just owning it was a palaver, the sheer embarrassment that people wanted to talk about my Hasselblad more than my pictures, dodging questions by saying 'it's just a camera', the shininess of it when photographing in public, the stupid price of every accessory, and then the internal debate whether to show I was using 'a proper camera' by printing the full frame with the registration notches. I sold it and bought a Mamiya C330s, ah the relief....