Here's an interesting twist from the above post: "No wait, let's use PGP just a little bit longer. Jacob Appelbaum recommends to use PGP over Pond instead of over E-Mail. Indeed, in that case most weaknesses listed above are no longer a problem. Also you don't depend totally on the safety of Tor and Pond..."
My conclusion: use several nested layers of techniques if you want security (don't use just PGP, for example). This should be true not only for secure personal communication but also for designing secure software.
A couple of you have mentioned the traceability of typewriters as a downside. In fact that is a primary reason for using them. Like a gun, every typewriter has unique quirks that can be used to uniquely identify content typed on that machine. In the event of a leak, the Russians (for example) hope to use this forensic info to identify the leaker. The general knowledge of this acts as a deterrent.
Moreover, it's a common strategy to step back a few levels of tech innovation to defeat an opponent. Bayonets. Cops on bike patrols. Hipster bike messengers. Insurgents originally used cell phones to detonate IEDs (Iraq or Afghanistan), but then they had their phones jammed by the USM--so they switched to string. Al-Qaeda uses hand written notes, hand-delivered.
FOIA laws and e-discovery also cause people to go low tech. To get around FOIA requests government officials have simply stopped using email (others used webmail under a false name). And in my last start up, serious issues raised on email elicited a response by phone in part to avoid potential e-discovery in a lawsuit. That kind of shady behavior caused me to leave.
My conclusion: use several nested layers of techniques if you want security (don't use just PGP, for example). This should be true not only for secure personal communication but also for designing secure software.