I have written before about how overzealous design of cryptographic protocols often results in their non-use. Protocol engineers are trained to be thorough and complete. They rankle at leaving in vulnerabilities, even against the most extreme threats. But the perfect is often the enemy of the good. None of the various protocols to encrypt E-mail have ever reached even a modicum of success in the public space. It's a very rare VoIP call (other than Skype) that is encrypted.
The two most successful encryption protocols in the public space are SSL/TLS (which provide the HTTPS system among other things) and Skype. At a level below that are some of the VPN applications and SSH.
TLS (the successor to SSL) is very widely deployed but still very rarely used. Only the most tiny fraction of web sessions are encrypted. Many sites don't support it at all. Some will accept HTTPS but immediately push you back to HTTP. In most cases, sites will have you log in via HTTPS so your password is secure, and then send you back to unencrypted HTTP, where anybody on the wireless network can watch all your traffic. It's a rare site that lets you conduct your entire series of web interactions entirely encrypted. This site fails in that regard. More common is the use of TLS for POP3 and IMAP sessions, both because it's easy, there is only one TCP session, and the set of users who access the server is a small and controlled set. The same is true with VPNs -- one session, and typically the users are all required by their employer to use the VPN, so it gets deployed. IPSec code exists in many systems, but is rarely used in stranger-to-stranger communications (or even friend-to-friend) due to the nightmares of key management.
TLS's complexity makes sense for "sessions" but has problems when you use it for transactions, such as web hits. Transactions want to be short. They consist of a request, and a response, and perhaps an ACK. Adding extra back and forths to negotiate encryption can double or triple the network cost of the transactions.
Skype became a huge success at encrypting because it is done with ZUI -- the user is not even aware of the crypto. It just happens. SSH takes an approach that is deliberately vulnerable to man-in-the-middle attacks on the first session in order to reduce the UI, and it has almost completely replaced unencrypted telnet among the command line crowd.
I write about this because now Google is finally doing an experiment to let people have their whole gmail session be encrypted with HTTPS. This is great news. But hidden in the great news is the fact that Google is evaluating the "cost" of doing this. There also may be some backlash if Google does this on web search, as it means that ordinary sites will stop getting to see the search query in the "Referer" field until they too switch to HTTPS and Google sends traffic to them over HTTPS. (That's because, for security reasons, the HTTPS design says that if I made a query encrypted, I don't want that query to be repeated in the clear when I follow a link to a non-encrypted site.) Many sites do a lot of log analysis to see what search terms are bringing in traffic, and may object when that goes away.