"Opportunistic encryption prevents passive attackers from being able to collect data, they will either fail or be forced into an active role (if they are positioned and financed to do so). While many talk about increasing the cost of surveillance for groups like the NSA, I doubt that will create a substantial impact - we know that they are both active and passive today. So they are positioned for active attacks when they so desire, though there may be some reduction of monitoring lower value targets due to increased complexity / resource demands."
The Wikipedia definition: "Opportunistic encryption (OE) refers to any system that, when connecting to another system, attempts to encrypt the communications channel otherwise falling back to unencrypted communications. This method requires no pre-arrangement between the two systems."
A blog post by Patrick McManus, a Mozilla dev, goes into further detail:
"OE provides unauthenticated encryption over TLS for data that would otherwise be carried via clear text. This creates some confidentiality in the face of passive eavesdropping, and also provides you much better integrity protection for your data than raw TCP does when dealing with random network noise. The server setup for it is trivial.
These are indeed nice bonuses for http:// - but it still isn't as nice as https://. If you can run https you should - full stop. Don't make me repeat it :) Only https protects you from active man in the middle attackers.