Before the rant, let me clarify the terms, because the meaning of Email is quite overloaded. There is the protocol SMTP, which is used to send email. There are the protocols POP3 and IMAP used to fetch email. There is the data format RFC561 plus extensions.
There is no problem with the protocols, since they could be (relatively) easily changed. The situation is quite complex for admins, because of the various addons like PKI, but there is nothing substantial to fix here.
The data format has one big problem, because it is not extensible in a seamless way. For example, we would like to add PGP encryption, so we can choose between inlinePGP and PGP/Mime. When an unknowing users reads such an email, he will either wonder about the garbage text or the broken attachment, respectively.
The point is that multipart-mime cannot specify to "ignore this part, if you cannot handle it", so the client shows unknown mime-types as attachments.
This is the fundamental problem with email, which can only be fixed by fixing all client. In other words, it is impossible.