[continuing from previous email and changing email header per bastien's request to have separate threads/conversations per suggestion.]
another useful change imo would be to allow categories to have their own faces, selectable by regexp (so "^m-.*" could be yellow). similar to tags and todo kw. then you could tell them apart without things like having to use the < command. in the agenda, currently (i think) they use the same face as the whole line in the agenda.
