Some people go to the extreme of saying they don't need tags OR folders in Gmail because the search is so good, but that's missing the point - there are two use cases in play with email:
- Finding a specific email that you are looking for - search works well here
- Finding all emails about a particular subject - it's pretty difficult to come up with search criteria to do this for you
Tags allow you to categorise your mails, and most importantly, use multiple tags on the same email, which you cannot do with folders. This is a key feature.
Gmail just added some hierarchy to their tags (mentioned by ReadWriteWeb) but its a fairly hacky looking 'labs' feature from what I can tell. The ReadWriteWeb article seems to get it a bit wrong really.. I see no evidence of giving up on tagging - the feature simply allows you to organise your tags in a hierarchy, cleaning up your list of tags.
Proper hierarchical tagging is something I would like to see, and not just in Gmail - how about Evernote and other apps? In fact Evernote might be an easier example..
In Evernote they have a similar nod towards hierarchy that actually does nothing but organise your tag list. I have a tag hierarchy for my house project, 'House -> Top Floor -> Office' etc, but I have to remember to tag an item relating to my office with 'House', 'Top Floor' and 'Office' to achieve what I want. What is it I want? To be able to click on 'House' and get all items relating to my house, or 'Top Floor' and get all items relating to the top floor. If I just tagged with 'Office' the fact that 'Office' is actually a tag in a hierarchy is completely lost other than where I click on the tag name.
Do any applications implement a decent hierarchical tagging system?