Two of my "favorite" features it didn't even touch on. You can have nested comments:

foo(one(two(three(four(five(six(seven)))))))@example.com

This will actually fail on that big email regex that gets copied around (originally from Mastering Regular Expressions in 1997), because it can only handle comment nesting to a depth of six. It is actually possible to do indefinite nesting now with recursive regex, but it was developed before that feature existed.

RFC822 also allows routing addresses through multiple servers:

<@foo.example.com@bar.example.com:123@example.com>

But this is almost always denied on modern email servers because it was abused by spammers.