I must've said this a thousand times, but I still believe it to be true: Google should make "Nexus" a certification. I know that there are already some requirements if you, as a vendor, want to use Google Play Services on your device, but this would be a more strict enforcement of a set of rules, that the Nexus devices have already historically followed.
The main issues would be no vendor skins, but uninstallable custom launchers, camera apps and the like should be permitted, and some kind of OS upgrade guarantee for a certain period and a limit on certified devices per vendor per year.
It might not be the best comparison, but on our computers or laptops, whether we're using Linux, Windows or MacOS, we're used to things looking (and working) basically the same if we buy a new device with the same OS, except, of course, if it's a newer version.
If you buy a Windows laptop from Dell, you get pretty much the same software experience that you'd get from Toshiba, HP, Acer or whatever. You might get some crapware, but your Samsung laptop won't have Touchwiz instead of the usual Windows UI.
Additionally, no matter which brand you buy, your machine will periodically update with the latest security patches and so on.
I don't see why we shouldn't expect the same thing from our phones.
I've even found myself considering, that I'd pay for Android version upgrades, if that would guarantee that my phone was supported beyond the two years, that is currently considered the lifetime of a Nexus device.
The benefit to the user of the certification, would be ease of use and peace of mind when buying a new device.
I've stated many times, that the diversity of Android devices is simultaneously the greatest strength and weakness of the ecosystem.
If you're willing to spend a good deal of time reading reviews and doing general research, it's very likely that you'd be able to find an Android phone that you'd be happy with. On the iOS side of things, if you want the best phone, you just walk into the store and ask for the latest iPhone, decide on size and then walk out again, confident that you got the latest and greatest. Samsung has had some success with a similar approach with their "Galaxy S" line, but personally I think we could strike a nice balance between no choice and too many choices with a yearly lineup of Nexuses from different vendors.