So the functionality does exist 'kind of' with things like MailActOn and the build in filters. The same could be said for Folder Actions and stuff like that. No the part that is missing is the same thing that Hazel does, it watches, it waits and it acts. What I am picturing is something that would, for example, watch the inbox and if there are messages that have been left unread for two days, moves those messages to a mail folder like "Unread Inbox" messages that have been read that are older then a day get moved to "Inbox Archive" Perhaps it goes further and could trigger defined mail rules, add MailTags, color messages based on age.
The thing that makes Hazel so useful for me is that I don't even realize that it's working, yet I now can look in folders tell the relative age of things by inspection., my desktop is clean (even though I might have apps I'm developing spitting log files out there during the day, I know they will get cleaned up)
It is the idea that there is something managing the time relative information in mail to filter 'later'. I haven't found anything that operates like Hazel in the background keeping mail organized like Hazel can keep my filesystem (or at least parts of it) managed
Sam D