Is it just me or is it that most progress nowadays seems to be just rediscovering features that old specialized apps like emacs have had for decades now?
Seriously, conversations... GNUS has had that for ages and it is a very nice feature.
Threading has been around forever, but I don't think any other email client has Gmail's "conversation view" (I'm kind of surprised that nobody has copied it yet, actually).
http://radar.oreilly.com/archives/2007/03/sfearthquakes-o.ht...
http://www.wingedpig.com/archives/2007/03/back_to_the_future...