Saved searches
The "search folders" or "saved searches" feature from Evolution is pretty handy. It generalizes the "merge multiple inboxes" feature mentioned in #108 (closed). One use case would be to merge mailing list folders from different accounts.
Personally, I have found saved searches to work better than server-side filters in some cases. For instance, I don't want to use any filters that archive things on Gmail, since this interferes with the mobile and web interfaces, which I still occasionally use. As a consequence, I need some client-side filtering to organize my messages.
This feature depends on #656.