Just a random thought as I start posting again: I want an easy way to clean up thousands of old emails quickly. Sure, I could leave them around, but frankly it just bugs me to know that I'm maintaining a heap of digital compost some of which I will never, not in a million years, find useful again.
The surest way would be to quickly tag any email I read and choose not to delete with some indicators about why I didn't just ditch it immediately. Evolution, the Linux Outlook-a-like, gives me a little of this via its Followup feature; Thunderbird didn't have a plugin to help with this when last I looked.
Still, there should be some helpful methods, such as a list of all the old mail's senders along with a count of how many they sent and expandable to see the titles. Let me easily select all or some of any sender's emails for deletion (for UI, think packages/subpackages in an installer). Others might be grading the importance of the old email according to length of thread, number of times I replied in the thread (indicating I found it important) and so forth.
And yes, I've got a lot of these to go through. =\