I have a suggestion for Windows Mail. It's importance is not very high though, but I thought it was nice to share with you.
In some situations, I have to attach to a message a file which I will never use after that. The file may be deleted then. Currently I have to do this myself, but perhaps Windows Mail can do this for me when I can specify this in the Open File dialog.
This will become handy in a lot of situations, and also applies to Outlook and IE when I send attachments.