The am/pm "feature" on any digital clock, but especially on an alarm clock. It makes it infinitely more likely that the alarm or time will be off by 11+ hours. My microwave forces me to press <Clock>1234<Start>2<Start> to set the time to 12:34 pm. Thanks, Kenmore, I needed that "PM" light to know that it's daytime.
24 hour clocks shouldn't have this problem, I guess. Looking at some pictures of 24 hour clocks, though, I don't think I would be able to get used to the analog ones that divide the circle into 24 hours.