Does Nexus 6 sold by AT&T differ from one sold on Google Play in any way? What are the differences between the Google Play Edition of the Nexus 6 and the AT&amp;T version? Does it have any AT&amp;T "bloatware"? Will its updates come from AT&amp;T instead of Google? Is it locked to AT&amp;T's network?
Nexus 6 models sold via AT&amp;T are reported to have these differences (source 1, source 2, source 3):


It is SIM locked to AT&amp;T
There is an AT&amp;T logo on the back of the phone
The phone displays an AT&amp;T branded boot animation/sound on startup
It performs a check to ensure your current plan allows tethering before it will allow it to be activated
Several AT&amp;T ringtones have been added
After the initial setup, if an AT&amp;T SIM is inserted/activated, it will download and install several AT&amp;T apps automatically. These can evidently be removed, without root.


It has been speculated that updates will still be received on-time because the AT&amp;T extras are on a separate partition (mounted at /oem), but this has not been confirmed, to my knowledge. With root, you can remove the ringtones/boot animation from the /oem partition. You would need to obtain an unlock code in order to use it on a different network.