Well, it was stated that the Captains were the military assets of Soul Society. So at least we know there was no mincing of words. They are significantly stronger than those around them and they are generally "once in a hundred years" type of characters. It almost always requires the use of a bankai to be part of the Gotei 13 with the exception of the 11th squad.
Basically, they are the "super-weapons" of SS.