From the beginning of the Nexus line, Google has always been good at making sure the Android experience on Nexus devices remains in its purest form. The idea is that stock Android shows the full capabilities of the OS on Nexus devices without any customized features. If you take that stock Android and put it on another device, it should have the same features as any of the Nexus devices. Unfortunately, that is no longer true with the Nexus 5. It turns out there are a few custom features found on the Nexus 5 that will not be available if you install Android 4.4 KitKat on other devices. Here's a quote with more of the details,

The Android used on the Nexus 5 is pure Google, but according to J. R. Raphael of Computerworld, Google has confirmed that a few features on the Nexus 5 will not be part of KitKat distributed to other partners. These features include the home screen integration of Google Now and the ability to initiate a Google search by speaking "okay, Google" while on the home screen.

This is significant as the version of Android previously used on Nexus products has been the raw version that OEMs get for modification. With exclusive features on the Nexus 5, Google is behaving like its partners and customizing Android for its own products. It may give reasonable excuses for such customization, but the fact is now stock Android is fragmented, and by Google.
It's possible that some of these custom features can be implemented on other devices, but it's still a strange change of pace to see Google basically fork their own OS on their Nexus device, even if it is ever so slightly. What do you think of this odd move by Google which seems to increase fragmentation?

Source: ZDNet