Your intended point about pigeonholes can sometimes backfire within the context of email. Consider a Gmail user who sends a query to a number of different recipients, expecting unique replies from each. Gmail, because of the like headers, will group all of the responses into the parent conversation. Let's say the users tags the first response with a label corresponding to that sender, and then the second response comes in. At this point, the user is at an "oh shit" moment, finally comprehending the implications of Google's labeling of conversations rather than individual messages.
You need to be able to specify both parent and child tags to be able to facilitate search and give users something that isn't too confusing re: organization.
I understand the pigeonhole filing example, but I quit "filing" years ago. It's tedious. Tagging also takes too much thought, particularly at recall time: did I tag that "development" or "programming"? I just use search.
I have a script that monitors my desktop. Any file not modified within the past 8 hours is moved to a folder dated for the end of this week. That gives me 52 folders a year, granular enough to find a file manually if I know roughly when I dealt with it.
My desktop stays clean except for what I'm actively working on, and I don't look for files hierarchically or by category/tag. To find something, I use Spotlight search. That's faster than navigating a folder hierarchy would be even if I knew exactly where the file was.
That's also why I love Gabor's reMail on the iPhone.
> and then the second response comes in. At this point, the user is at an "oh shit" moment, finally comprehending the implications of Google's labeling
I am having trouble understanding what the problem is, here. I also suspect this may be a Gmail problem rather than a tagging problem.
It's tagging a conversation rather than tagging an email.
I frequently put different emails from the same conversation in different folders, because they contain different types of information that I need to do different things with.
You need to be able to specify both parent and child tags to be able to facilitate search and give users something that isn't too confusing re: organization.