The Android 4.4.4 update for the Nexus 5 contains mostly security fixes, including a major one for issue CVE-2014-0224 affecting the Open SSL protocol. However, these are just the changes visible in the AOSP code; Google could be using the opportunity to release updates to the proprietary bits of the Nexus 5’s firmware. We know that the update changed the baseband version to 1.16, up from the 1.13 version from Android 4.4.3, so if you had troubles with connectivity on your device, this update may help solve those issues.
On a side note, Android 4.4.4 does not appear to patch the Towelroot root method that celebrity hacker Geohot released earlier this week, which is based on the CVE-2014-3153 vulnerability in the Linux kernel.
If you can’t wait for your over the air update, you can manually install the OTA file using the file captured by XDA-Developers member Sucro and available on this XDA thread. The update is incremental, meaning that it will only work if you are on the most recent stock version of the Nexus 5 firmware, which is Android 4.4.3 KTU84M. Sideloading the OTA file will bring the firmware version to KTU84P.