You know what else has a great design on a plane? The turbofan engines. In fact, give me a blueprint for a 757 and I can probably find 50,000 things with better design than the bathroom lock. I have a special fondness for the RAT:

http://en.wikipedia.org/wiki/Ram_air_turbine

and if I had to choose between the bathroom lock and this set of technologies,

http://en.wikipedia.org/wiki/Ice_protection_system

...well, I just don't know. But no one adores the trivial like 37signals. Of course, you say, "They are focusing on user-centric design." Well...in that case, the door-light combination doesn't seem as good as a motion-detector-door-light combination, which would turn ON with motion, stay on when LOCKED, and turn off when UNLOCKED. That didn't take an engineering degree to come up with. Where's my kudos?