My Nexus 7 (2012 Wifi) got stuck in it's boot screen (4 circles animation) after the legit automatic OTA update of 4.3 to 4.4 (KitKat), and wouldn't even let me switch it off ...
To fix this issue, download Nexus Root Toolkit (free) and follow these steps:
1) Power Down and go to recovery Mode
If you can't seem to switch off you Nexus, keep the power, volume UP and DOWN buttons pressed simultaniously for up to 20 seconds, and it wil power down. I counted 19 seconds before the shutdown, others claim up to 20 seconds. The "broken Android" picture will appear ... you're Nexus 7 is now in recovery mode.
2) Start Nexus Root Tolkit and do a factory restore
After starting Nexus Root Toolkit you'll be asked what model Nexus you have and what Android version you're running. If not sure about the andorid version, simply pick *.* (last in the list).
Next, connect your Nexus via the USB cabe to your computer and wait a few seconds so Window scan recognize your Nexes.
Choose now the option "Soft bricked / Bootloop" under "Back to stock".
Next, click "Flash to Stock and Unroot" if your Nexus is showing the tipped over Android, and click "OK".
Nexus Root Toolkit will pick the latest version suitable for your Nexus automatically (at the time of writing this: NAKASI-GROUPER: Andrid 4.4.0 - Build: KRT16S voor de Nexus 7 Wifi van 2012), make sure it's selected and make sure "Automatically download + extract the factory image selected above for me" is selected as well and click "OK". The download process will now start.
After download has been completed, the download will be verified and extracted. A message will appear asking you if you're ready, click "OK" and a DOS window will open allowing you to follow the progress ... be patient. Messages about missing .sig files can be ignored. It took about 3 minutes on my setup.
After completion the Nexus will boot and might even look like it's boot loping again - be patient this can take up to 10 minutes ...
After that: you're Nexus is free again ...!