I love the tagging via labels – very useful. What would be really cool is the ability to attach a note to an existing email – something to remind me of the email – why it’s important, whatever.
For instance, I just got a “Welcome” email from a new web site. I would love to be able to attach a note as to why I thought they were interesting, a hint to my logon/password, whatever. And index it. Make is searchable based on search string, labels, etc.