Google has pushed Android 6.0 source code to AOSP (Android Open Source Project), and released factory images (MRA58K) for Nexus 6 & Nexus 5 smartphones, Nexus Player, as well as Nexus 9 and Nexus 7 (2013) tablets.
You can retrieve the code with the usual repo tool:
repo init -u https://android.googlesource.com/platform/manifest -b android-6.0.0_r1
Some of keys changes are listed on Android Source microsite and include Doze and App Standby to save battery, new authentication interface sich as Fingerprint, Gatekeeper, and Keymaster, adoptable storage, Bluetooth stylus support, and more. If you want a more detailed changelog at the source code, Opersys got you covered by listing all differences between 5.1.1_r24 (LMY48W) to 6.0.0_r1 (MRA58K) with newly added components, removed components, and modified components.