Many systems use tags these days but I think they differ in a single but important way: is the tag name allowed to include a space and if not, how are multiple words combined? One way would be to use underline characters or no characters at all. I do dislike both of these methods as underlines are ugly and it might be a difference if two words are used separately or combined. So considering the space as a part of the tag would be the way to go I guess.
However, if I enter tags to a URL posted to del.icio.us I have to separate tags using spaces. On the web however, separating keywords with a comma is much more common.
I don’t know how del.icio.us handels this, but flickr allows spaces in tagnames when you quote them. e.g. „foo bar“ is one tag.
I think del.icio.us handles it like flickr with „bla bla“… as posted above.
I have a question about yours.
Why does it matter so much?
Technically, there is no reason not to allow spaces. Tags can be arbitrary strings as long as there is a way for seperating them, which can (but does not have to) be based on separator characters (like space or comma).
The user experience is definitely different when spaces are forbidden, but i don’t think this is really important because the user experience of tags is pretty technical anyway.
I prefer systems without spaces in tags, but consider this a question of taste, not reason.
that’s the reason why i prefer open systems like scuttle or unalog, which are available in source. i can run them on my own server, i’m not depending on any service provider and i can add features i like to have.