Nexus 9, Nexus 6, and the Nexus Player. Also, Lollipop. (cred: PC Mag)
There's a few ways to cut this up in the way that I analyze devices, namely Android phones and tablets: System, Aesthetic, Features, and Future (SAFF). My analysis looks at the devices and sees what they do with Android and how it impacts other manufacturers, like Samsung, LG, and Sony.
System: What do the Nexus devices do with Android?
The Nexus devices are more than just a new way for Google to show off the newest major Android version; they also act as a benchmark for companies like Samsung and Sony to look at as a reference. They act, necessarily, as the standard for what the next version of Android should do and what should be using it. What processor is required? What kind of camera is best for an Android device? What should be the "core" of the system? These questions are answered with Nexus devices running the most current and unadulterated version of Android. No manufacturer UI, no special gimmicks or tricks. Nexus devices show what Android should be at its heart.
Aesthetic: What does it all look like?
Android introduced the Material Design aesthetic language with Lollipop, making clear certain standard for app developers looking to design their apps to work with Lollipop. However, whether manufacturers will follow through is a different question. Samsung will reportedly still take a more vibrant approach in the overall UI, but LG may only touch it up a bit. Material Design is at Android's core of visual appeal. Manufacturers need to understand that not everyone wants all the extra features or flair of their UI; maybe allow users to choose what they want when they boot the device or offer the UI as an add-on app in an exclusive app store? The Nexus devices show Material Design as a reference again, but also say "follow our example" like the pesky older sibling in your family (sometimes).
Features: What can the Nexus devices do?
This question really can answer itself, but the fact of the matter is that Nexus devices always set the standard for what Android devices of all flavors should be able to do. Samsung has the heart monitor, but it still follows Google's standard of app folders, widgets, etc. Likewise, HTC may have had Blinkfeed for a while now, but who originally had NFC payments and cross-app communications? Nexus devices. Even then, OEMs (Original Electronic Manufacturers) have had to take a few cues from other devices (*cough* Nike Fuelband) and app developers (*cough* Flipboard). Nexus itself can be guilty of this, but made it integrated naturally not too long after.
Future: How will the Nexus family handle future Android versions and what do they allow for that future?
The crystal ball may not be clear for technology, but the future can still be predicted. The Nexus devices often act as the proverbial canary in the coal mine for many features and functionalities of Android. If it works, it'll be standard in future Android devices and may be used eventually by a certain competitor years later (*side-eyes Apple intensely*)! If it doesn't work, it's easily erased. Presto! If people like it or want it, it's included! If they hate it, it's gone!
So how will manufacturers take to Lollipop? Will they finally stop preloading crap apps? What about network carriers? We'll see soon!