The only features I need an email client to have is extreme speed and nice search. No desktop clients for mac have both right now. This isn't fast enough but i'll keep it installed and check for updates in a month.
If you want a commandline mail indexer, Notmuch has both. It's like Mutt but it uses a fulltext database (Xapian), so it's super fast (eg. subsecond fulltext searches on my 80k emails).
Command line though, and you need to pick a frontend. Definitely not for everyone. http://notmuchmail.org/
