Tons of New Features
Expect the Android 6.0 update for the Nexus 5 to be a big update, maybe even bigger than what you're expecting.
Google only outlined a small portion of the Android 6.0 Marshmallow features at Google I/O. The update's change log is actually far more extensive than some people think. While it might not be coming with a huge list of visual changes, it's coming with tons and tons of enhancements for the Nexus 5.
You should expect this update to have a profound impact on your Android experience. Hopefully a positive one. Do yourself a favor and get familiar with Android Marshmallow and its changes ahead of the roll out date.