Apple Mail is nice and clean, but it still lacks a lot of the features I want in an email app.
Apple Mail now has rich text, but it still has no ability to label your emails in different ways other than to flag them. (See how many "Flagged" emails I have?)
Apple's Mail app also doesn't sort your emails in ways that other email apps do — like when it detects an email contains a plane ticket, or a receipt. Edison Email, a free mail app I've been using for the past several years, can automatically categorize your emails into "subscriptions," "travel," "packages," "bills and receipts," and "entertainment." I'm still waiting on Apple to implement a feature like this, so I can quickly find my plane ticket receipts, for example.
Also, if you want to tweak your Mail settings, you can't do it in the Mail app; you have to visit the Settings app and scroll down to Mail.
Overall, Apple could have done much more with Mail. I was excited to re-open this app for the first time in years, but came away disappointed. I'll be sticking with Edison Email for now.