That really is the sign of a great conductor -- someone who envisions the sounds and interpretation he wants and knows what to do to get it out of his performers. It can seem nit-picky, but so worth the effort when the result is so refined and great. I have sung in choirs where the conductor really could not isolate problem spots and just had us plod on oblivious to our problems. I have also sung in groups where the conductor could only really hear her own voice part, mostly melody. She would spent inordinate amounts of time correcting that one part, but not even be aware of problems in the other parts. While it is true that the melody, or often the highest part, is most likely all the audience pays attention to, they can still tell when something is not right with the entire group. They might not be able to pinpoint the problem, but they will very freely give their opinions when something just did not sound right.
Like a good cook who can isolate and tell you the component ingredients in a dish just from taste or smell, so a good conductor can isolate problems in his choir or orchestra just by listening. And then he will quickly proceed knowing just what to do to balance or correct the issues. That takes years of perfecting musicianship skills and lots and lots of listening! But the talent of giving little cues just before the problem is to come in the music, that is really something! That takes thinking ahead and planning just what to do to get the best performance out the of the singers. A conductor who can do that is definitely not thinking of himself, he is only thinking of what is best for the music. And when the chorus watches and responds well to those timely reminders, a great performance is bound to occur. It is a truly amazing conductor who can make that happen!