Fragmentation has long been the bane of Android. The one way in which iOS was definitively better. All the recent iOS devices get updated, they all get the benefits when new features are added. They are all "pure" iOS experiences. Essentailly, every iOS device is like a Nexus device.
A long process
Slowly over the past few years, this problem has been going away. Many Google Apps that were originally packaged with the OS started to become available in the Play store so that everyone could have the latest version, instead of waiting for OS updates for your particular device. It started with things that are obviously apps, such as GMail, Maps, Calendar. But soon it was spreading to more basic features. The browser. Even the stock keyboard is now available as an app in the Play store.
Next was the Google Play Services. When I first saw that app on my phone I was mad. My first thought was, "what the hell is this? How did an app get installed on my phone without my knowledge?" I immediately uninstalled it. When I saw there again not much later, I took to the internet. And boy, is Google Play Services one of their best ideas ever.
As new features were added to Android, everyone had to wait for their phone to get the next version of the OS. But now, they were instead adding the features through this app that everyone had. No more waiting for the painfully slow update from your carrier for your specific phone. All sorts of cool new things were added through this app instead. Notification syncing between devices, single sign-on functionality. Take a look at the new features added in the last few versions of Android. Those lists have gotten pretty small. Some things still need an OS update, but most features are added through that app now, so even old devices can benefit. One of the biggest improvements in Kit Kat is reduced RAM usage. This will improve performance, but isn't really a "feature".
Crapware and Skins
"Well that's great, all the new Android features are added to old devices. But what about all the junk on my device that I don't want?"
I'm glad you asked that, nameless person I made up! A while ago Android added the ability to "disable" built in apps. This meant that if you don't use Google Play Books, you could essentailly remove it. A disabled app is pretty much the same as uninstalling it. It still takes up a tiny bit of space, because it's still there and can be enabled later. But it's gone from your app drawer, it can't run or do anything, it's essentially gone. Don't want that "VZ Navigator" app that Verizon gave you? Just disable it! Technically you still can't uninstall crapware, but this is close enough.
So what's left then, to keep the devices fragmented? Skins. Touchwiz, Motoblur, Sense, whatever LG calls theirs. These customizations each company makes to Android are supposed to be a good thing. They add features, and make the devices from that company stand out from the pack. But as almost any Android fan will tell you, these skins are in fact a bad thing. Mostly useless, gimmicky features that no one uses, and end up slowing down the device to some extent. Sure you could always install a custom ROM, but a large majority of Android users don't even know what that means, and doing that isn't really an option. The next best thing is custom launchers, which can mostly replace the skin of the manufacturer, but still isn't a stock Android experiene...
The final piece of the anti-fragmentation puzzle is here! Google has now released the "Google Now Launcher" which can be downloaded right from the Play store. Now if you want as close to stock an experience as possible, you don't have to download a launcher that attempts to emulate it, you can download the actual vanilla Android launcher. It's not exactly the same as installing a custom ROM with a stock version of Android, but this gives the average Android user an easy way to have an almost pure stock version of Android. Don't like the weird launcher that Samsung or HTC gave you? Just install this launcher and it's almost like having stock Android! Now, just about any Android device can be essentailly running stock Android with all the latest features, without waiting for any slow updates from your carrier!
Of course, when I say "now" I really mean "soon", because at the moment the Google Now Launcher is available only a select few devices. But it's expected that in the near future that will expand, and I think it's safe to assume that most devices going forward will be able to install this. It's been a long, painful journey, but Frodo has finally thrown the One Ring of Fragmentation into the fires of Mt. Doom.