URLs (Am I the only one who likes to know where I'm going when I click a link?)
@handwaving one thing Thunderbird does that I really like; when an email includes a link with the link text being an URL, and the link itself points to a different URL (usually tracking bullshit for the URL the text shows) Thunderbird asks you which URL to go to, so you can avoid the tracking.
@chopsstephens That's a pretty neat feature!