Ken Norton addresses in some detail a common UI problem with tags - how to handle delimiters, i.e., how to allow multiple tags without forcing people to mash terms together like “sanfrancisco”.

There’s a bit of an irony here. The openness of interfaces like del.icio.us (where you’re just given a text input box and the unclear clue of “space separated) has lead directly to tag uptake. But that uptake is still limited, and, I would argue, limited by that openness–there’s a large part of the user population that, when faced with an empty text box and little help, are daunted, and enter nothing.

There are many problems to be addressed in tagging UIs. The challenge is not getting in the way (tagging works because of its low “cost”), while providing enough cues and structure so that people feel confident in using it.