I guess that, as a human factors engineer, I am a little over-sensitive to poor design. But what things have you come across that really bug you, simply from poor design? This is not only a question for the professionals, but also the "Self-loading cargo" and anyone else who is unfortunate enough to come across these things.
A couple of examples for you:
I frequently fly at night as a passenger on a 738. Being the "hours of darkness", the FAs dim the lights for take-off. Of course, everyone reaches up to turn on the overhead light. However, the switch for the light is not lit, and EVERY SINGLE time it happens, half the passengers inadvertently press the "FA Call" button.
In the Harrier GR7 (it should be the same for the AV-8B), the trim adjust switch is right next to the selective jettison button. You would be amazed at the number of baggage pods, external fuel tanks etc that have been dropped on unsuspecting (and probably innocent) souls.
There are plenty of others, I am sure - so share them with us!