One big point for me: Except for Airmail, none of the new "minimalist" mail apps do plaintext. They all force you to send as HTML. I would love for "rich" email to work, but it's not nice the way current clients do it. In particular, I think it's incredibly presumptuous for the sender to specify which font the recipient should read the text in.
My favourite mail client ever was Sparrow, until Google killed it.