If you've been using Mutt, the concept of multiple recipients itself is a much harder process of discovery :)

Ironically, I am moving from Thunderbird to Mutt (NeoMutt actually). I find the elimination of HTML means it's actually easier for me to not get distracted by stuff I dont care about for email, which is interaction with real human beings, who are unlikely to be typing complicated HTML messages. Besides, with the right config (I'm still working on it), the HTML message is 1 shortcut away.