ANewMusicalBrain: "If you look at the whole show like a clock. 1 being the beginning and 12 being the end, 11 o'clock comes right before the closing of the show." This has always been my understanding of the term. It doesn't have to do with actual time, but with the song's placement in the show. My idea of the perfect example of an 11 O'Clock number is "The Miller's Son" from A Little Night Music. It has nothing to do with who is the big star and it's usually not the last song. It is the BIG (usually solo) number near the end of the second act that everything has built up to.
While it's difficult for me to define it exactly, it either comments on or illuminates what's come before it, or it is the culmination of the buildup that come before it. It's often followed by another song that wraps everything up.
I haven't read the whole thread so I may be repeating (or contradicting) what's already been said.
Art has a double face, of expression and illusion.