It is indeed surprising that more of the productivity ideas in other realms have not been applied to tagging. Tagging is one of those things that is so popular that people tend to include it in their programs without thinking about it much, and as a result it could stand to be improved a lot.

Personally, I often use a "totag" tag on items that are partially or unorganized. Of course, I have to add this manually, so it's not ideal, but there you go. I totally agree that programs should institute a better system in place for collecting untagged or sloppily tagged items.

The only program that I know of and use personally that does something like this is Eagle Filer, which attaches an "unread" tag to absolutely everything that you toss at it. On the upside, this allows easier "inbox" editing. On the downside, "unread" is essentially meaningless for the use that I'm putting it to, and there's no option to change the default.