What really ended up being the dealbreaker was that the viewing and authoring experience for moderately marked up text was poor. Specifically: Quoting, bulleted lists, inline attachments, and maybe a few other basic formatting tricks. This kind of stuff doesn't end up rendering well when I view the mail in a terminal-based browser like w3m, and I didn't even try authoring (am I going to hand-code the HTML for every email I write?).
I wish we had something like Markdown for email--to me that would be the sweet spot between the mess HTML email has become, and plain text which sometimes does lack in its ability to communicate information.
Beyond that I would love any advice on how to use a terminal-based mail client but still be able to author and view some kind of useful representation of things like lists and quotes. Maybe with just the right configuration this can be done...