Is the SHIELD an Android device with a fully open source software? NVIDIA announced the Open Source Resources for the Tegra SHIELD device. Does this mean the SHIELD is a device with fully open-source software? How does it compare to another device, for example, the new Nexus 7 that will be released without the sources for the stock image, due to apparently issues with Qualcomm?
tl;dr NO, it is not fully open source.

From the site, SHIELD is described as a gaming platform based on the Android Jelly Bean OS. So, it uses Android, and you can create apps / games using Android. But most likely, they will also provide APIs that are usable only for the SHIELD device. If you look at the device, it has a lot of gaming controls or buttons that are clearly not available on a mobile phone.

The Open Source Resources section has a link to downloading (source codes, etc) and building your own OS image. So, it is Open Source (as of this time, anyway), but not completely. As Dan Hulme mentioned in the comments below, they still contain binary components, but are at least more open compared to Qualcomm.


  To assist the Android open source and modding community, NVIDIA has
  published the required source code and binary packages to create
  custom OS images for SHIELD


How does it compare to a Nexus 7 2013? The SHIELD is a gaming device with gaming controls. Meanwhile, the Nexus 7 is a tablet. [Guess: While tablets can have Tegra chips, it's possible that the SHIELD can do other functions that a tablet cannot. In this case, you may be able to see the game on Google Play, but it may not be compatible with your device.]

UPDATE 2013-08-09: To answer your comment (and this is purely my opinion), I think that Qualcomm-based devices would not gain much attention among developers / modders, given that they can't do much about it. Think Apple / iOS. In this case, Tegra would have an advantage and have a larger community building apps for their platform. In short, Qualcomm's loss.

Related news on Qualcomm legal issue:  


Legal Issues With Qualcomm May Prevent Factory Images For New Nexus 7 ('Flo') From Ever Being Published, JBQ Strongly Implies
Jean-Baptiste Queru quits Android project over Qualcomm drivers