My biggest pet-peeve is when they just ask for your email address, then on the next page inform you they've emailed you a one-time login code, and then you need

My biggest pet-peeve is when they just ask for your email address, then on the next page inform you they've emailed you a one-time login code, and then you need to hunt for the link in small text along the lines of "Log in with a password instead".