The user should be able to right click a message and select an "Instant Filter" option. This would bring up a version of the Filter dialog with "From", "To", "Subject" and "Body" fields filled in. The user could edit the values and save the filter without going through the Settings menu. Clearing a field would remove it from the saved filter. (Alternatively, check boxes could be provided to select the fields to be used in the filter.)
Differences from the current MWP dialog:
1) The Body field would have to be a text edit (text box) field, not a single line text input field.
2) The Entire Header field should allow for viewing and selection of all of the individual headers in the message. This could be done in a new window, or a tab on the dialog. (For that matter, the body text could be in a separate tab too.)
I'll leave the details up to the code wizards, I'm sure they'll come up with something cool. This is not a "gotta have" for the first release, but would be nice to have somewhere down the line.