Worst of all, Gmail especially isn't great about tagging messages as spam; a good chunk of messages sent from outside the Gmail network simply disappear. As a Gmail user, there is nothing you can do to verify that you're getting all of the legitimate mail traffic that is being sent to you.
Further, if you're trying to send any kind of business correspondence over email, you have to contend with a massive industry of scammers that are also targeting people with lookalike messages.
I love the email protocol, but from a broader service standpoint, email is terribly broken right now.