I think the name is a little deceiving... it there's "Quick Tag", where's the "Slow Tag"?
To me, it's just a graphical way of filling in "list"-type data fields. If you think of a multi-value (semicolon delimited) list-type data field as a container for "tags" (each value is a tag), the "Quick Tag" label makes sense.
When you're in "quick tag" mode, you can apply the tags (or rather, add values to the multi-value list-type data field) by simply clicking on the desired value. The hard way to add the "tags" would be to go to the properties window, find the appropriate field, and add the "tag" of choice (by appending a semicolon and the tag name).
For example, the People field for an image file might contain "Scott;Brian" (which means the file has the Scott and Brian tag). With quick tag, you can just click another tag (say Sabina), and voila, the People field changes to "Scott;Brian;Sabina". Or you could click the Scott tag, and voila, the People field changes to "Brian".
I hope this was of some help...
Scott-