What is lacking there is the ability to weigh up the validity of a tag based on how many people have used it for a given picture. Back to the instrument-concept, there are instruments that sound similar. Someone might tag a song “flute” even though most people know that it was played on recorder. A fully-fledged tagging solution would also take that into account so that I might see the track if I was searching for songs with flutes but only if my other criteria were so restrictive that the final selection was very small.
That in itself has drawbacks because, as any disgruntled citizen living under a democracy will tell you, the wisdom of crowds often compares unfavourably to the intelligence of sheep. However, at least (as also in democracy) there is the option to present evidence and arguments to win people round to changing their tags and, overall, the system should allow for pretty good if not completely accurate tagging of data.