The flipside of folksonomies:

Del.icio.us has another problem. It doesn’t allow spaces in tag names, resulting in the use of non-existing words: tocheckout is one of them. Thus, in certain cases, you only have the choice to assign flawed metadata to the content you tag, thereby lowering the value of your tagging effort.

Without a defined structure (for both users and services), the accumulated metadata is going to end up as a mess. If everyone is using their own tagging nomenclature (photo, photography, pics, pictures, snaps, etc.), and each service is collecting tags with different semantics (movabletype, movable type, movable+type, etc.)— in the end the collective data isn’t going to be all that easy to sort through and the value of tags is going to be lost.