Subject Lines might be considered irrelevant, but it seems absolutely discourteous if one has to open everything up to know what the damn content is saying. Nevertheless a Subject-line is always welcome from those who summarize the contents in a phrase, otherwise it is as good as having no subject-line at all!!! Ruling out CCs would be possible to a certain extent, but in a protocoled environment that means "please read it later and its just FYI" so don't bother right away. BCCs on the other hand are not replaceable by any means I can imagine ... possibly I am not thinking out of the box here, or is there any box at all to be considered in the first place!!??
References:
Will Facebook sound death knell for email?
Mark Zuckerberg defends Facebook's aggressive social strategy