Looking forward to better battery life with Doze in Android M. I'm the type that leaves my phone idle for the majority of the day so this feature would mean big gains for me in battery life.
The feature I'm most excited for in Android M is the granular permissions that put the power back into users hands. No longer would we have to put up with unnecessary permissions that are not warranted by some companies.
I am most looking forward to permission management, but I am also hoping for better RAM management and battery life. Lollipop still has some lingering issues that I hope get addressed in Marshmallow.