LED Notification lights have become more or less a staple within the Nexus devices. Before they were in nearly all Android devices, you could always be certain that a Nexus device would have an RGB LED notification light somewhere on the front of the device. They have generally been in the same spot over the years with each model, and with this year's Google Nexus 6 some had thought that there wasn't an LED light this time around. The past few Nexus phones have had the LED notification light inserted within the bottom bezel, in fact, it was in the same spot within both generations of the Nexus 7 tablet as well. On the Nexus One it was embedded in the trackball, which was also at the bottom of the face.
With the Nexus 6 there is no LED notification light on the bottom, so some though it not to exist. A user on reddit however had recently posted some images that show the LED notification light does indeed remain on the Nexus, it was just moved to the top within the speaker. As you can see from the images below the notification LED embedded there displays different colors in each image, after the user states they had messed around with Light Flow - Color LED to get it to change different colors for certain notifications. It's stated that the LED notification light is disabled out of the box, which makes sense due to the existence of Ambient Display on the Nexus 6. It should also be noted that you can enable the LED light for notifications yourself but you have to have a rooted device to do so.
If you have a rooted Nexus 6 then you can find the full instructions on how to get this going by following the Light Flow thread at XDA where the original post on how to set it up was posted. That thread doesn't contain the instructions on how to root your device though so be sure to brush up on how to do that if you haven't rooted the device yet and are wanting to. If you get things working share your thoughts on how you feel about the LED notification light as opposed to the Ambient Display for notifications.