sensible way. That bridge in Switzerland would probably be tagged with "Ginevra", "holiday", "aunt", and "Isabel" (plus a few prepositions).
You cannot be serious!?!
Well, I wasn't quite. But have a look at http://commons.wikimedia.org/wiki/User:Orgullobot/Welcome_log To say that 1 of 10 new users get it right - even nearly - is probably exaggerating.
Theoretically tagging is a great system but in the hands of a gazillion amateurs it will most likely lead to missing images, not found ones.
Yeah, and a Wiki doesn't work in theory either, fortunately it works in practice. Have a little more faith in our users. We don't have any "Holiday" or "Aunt Isabell" categories either. Claiming this will change when we use categories as tags is completely unfounded speculation (or FUD).
To make my point clearer: what we have almost works because errors and omissions (plenty of which are made) are easier to find. With tags, not so.