There are a lot of changes in the Google Compatibility Definition Document, which is a document that explains to OEM's how they can implement their own features into vanilla Android. This includes their UI overlay and apps that they install on top of that. Some of the changes are very welcomed to many of us, including the way fingerprints are being handled and the fact that every device must be encrypted.
Another change in the CDD is that manufacturers are unable to grant permissions to pre-installed apps on Android 6.0 Marshmallow. This means that users will need to grant them, themselves. Now this isn't a recommendation by Google, but it's mandatory. However there are a couple of exceptions here. One is that these are treated like any other app. So that means they will be required to ask the end-user for permission before doing anything. The other is when the pre-installed apps are replacements for default apps like the camera, browser, contacts, and dialer. Additionally, manufacturers must have an interface for the user to interact with these apps and allow them permission to certain parts of their phone and other apps.
This is actually a really great thing. This should hopefully cut down on the number of anti-virus and malware scanning apps that are activated on the first boot and have access to just about everything. These include Lookout, Norton, McAfee and plenty of others. So no manufacturers can't give their pre-installed apps permission to access everything before you've even signed into Google on a brand new phone. That's a great move by Google, and should help cut down on the bloatware being a huge issue. Although getting rid of it entirely would be even better.
These changes are new for Marshmallow, so devices that are launching with Android 6.0 Marshmallow, like the HTC One A9 coming later today, as well as devices that are getting updated to Marshmallow will have this feature. And there are a number of devices getting updated to Android 6.0 Marshmallow, starting with the LG G4, at least in Poland. Lots of great changes going on to make the platform even better.