With iTunes or thousands of bookmarks, I find that I can't "stumble upon" things anymore - I have to know what I am looking for, so that I can search it. I used to have my music simply organized in folders, and it was so much easier to browse (and still searchable).
Of course the iTunes style allows for new ways of stumbling across music, like "all songs that beginn with the Letter A", but I prefer the "old way" so far.
By the way Google does let you also tag (label) the bookmark. It's just that you have to press a different button to do that. Supporting both methods like Google does would work pretty well.
Yes, Google let's you label the bookmarks. However, I was curious about the preference for a flat list that some people seem to like. Would a bookmarking site that doesn't have tags nor folders be something that people would like to use?
http://www.google.com/support/firefox/bin/static.py?page=features.html&v=3