At the moment, I'm pretty excited by the announcement that "OK Google" is going to work across the entire phone, not just in Now or the home screen. The update is getting pushed out over the next week (I don't have it yet, grr).

"OK Google" will also work on the lock screen. When the phone is plugged in, it'll even work when the screen is turned off! I assume this is because unlike the Moto X, most phones don't have that dedicated chip in them, so battery life would be an issue if the Nexus 5 were always listening even when unplugged.