A Google+ thread has Michael Mahemoff asking what are the SEO pros and cons of switching your whole web site to SSL. The switch would change the URLs from http://www.domain.com/ to https://www.domain.com/.
Google's John Mueller said Google indexes https URLs just fine and ranks https URLs just fine. But there is no special ranking you boost you would get from having your whole site on SSL.
John said, "HTTPS-only sites are fine, there's absolutely no need to shy away from that if you implement it properly." He added, "there's certainly no penalty involved with running your site on HTTPS-only when done right." But does it benefit you? John said no, he said "just as there is no inherent disadvantage, there is also no ranking-advantage from using SSL in web-search, so I wouldn't use it in the vague hope that Google's algorithms will value the website more."
Then John offers advice if you do go the SSL route:
- don't forget the http->https redirect & other canonicalization things
- look into HSTS
- list the https site separately in webmaster tools (it's a different site)
- make sure the infrastructure can handle the higher load (SSL, caching, etc)
- check out the differences wrt. caching
Note, when making a switch to http to https, Google will treat it essentially like you are moving to a new domain. So make sure you handle everything you would, as if you would be moving to a new domain. Should you switch? I wouldn't at this point. But it depends on what you are doing on your web site.
Forum discussion at Google+.