They wear tighter rather than looser, and as they get tighter they get quieter. Other than the quietness there are no non-catastrophic symptoms. If the valves are too tight, the cams and the rocker arms will wear. If they wear too much, the hard outer layer will be worn through and the softer metal underneath will go away quickly. The parts that get destroyed are
#2, #3, #4 and #5 in this diagram. You can replace these - I have done it in a past bike (not an NT). But it's pretty expensive and a lot of work. Not to mention, all the metal that comes off them can't be good for the rest of your engine.
Not to scare you, though - this is not an item that happens quickly, and plenty of people get away with ignoring this maintenance item, for the entire life of their bike. The bike that I had to do this to, was a model with a known top end oiling issue, that was exacerbated by tight valve clearances.