[baddox]

The primary user-facing difference is that these "extensions" don't require additional installation to view content (that is, once all major browsers support them). From a technical perspective, there could other differences, like protections in place to sandbox the browser from the rest of the system, which is much better than requiring the execution of a closed-source proprietary binary.

[dave1010uk]

So if Flash player was built into all major browsers (like it is currently with Chrome) then that would remove the primary reason.

I thought EME was execution of a closed-source binary blob too. I also thought some browsers sandbox plugins (I could be wrong).

EME still seems to be just a new plugin specification, with a more limited API.