Email clients are available and usually installed by default on most OSes. The "From:, To:, Subject:" triad is well-understood by users and replicated on many other messaging systems.
Lack of Confidentiality
It's terrible for a messaging system to lack confidentiality, as confidentiality of correspondence remains a common rule in modern society, although the past decade saw anti-terrorist rule attack this constitutional right. GnuPG solves this for the content of the envelope: it encrypts the content, but leaves the envelope public.
Lack of Metadata Protection
Sending email is like sending postcards or flying colors: it's there for everyone to see from the starting point to the end point. Although it doesn't feel like it (and it shouldn't), sending email is more like shouting over a crowd than whispering to your neighbor's ear. We think it should instead resemble its analogical equivalent, now nicknamed "snail mail". Even better, the cover of the envelope should be scrambled in a way that does not require the intermediary transporters ("postmen") to learn anything from the origin or destination of the message. This is what the LEAP project is trying to do.