"... but the vast number of tags people can add is an even bigger one ..."
I agree! But I must confess, that Iīm also guilty of using to many keywords. For example: I have a picture with a very popular site in Venice, showing some gondolas and the Island San Giorgio Maggiore. My keywords are:
colin utz,europe,gondel,gondeln,gondola,gondolas,italia,italian,italy,la dominante,pier,san giorgio maggiore island,serenissima,unesco world heritage site,venedig,venetia,venetian,veneto,venexia,venezia,venice,fog,foggy,medieval,mist,misty,mole,overcast day,rain,raining,rainy day,tourism,travel,wharf,winter,www.colinutzphotography.com
Honestly, what would you search for, if you want a picture like this? I would search for Venice (Venedig, Venezia), gondolas, San Giorgio Maggiore. I would not search for "La Dominate" or "Serenissima".