Besides the nice hardware on the Nexus (although not quite as powerful as the Galaxy S III), you get almost instant OS updates. Other devices have to wait a few months (if they ever get the updates) since the carriers control the updates and have custom themes on top of Android.

I don't really care about "powerful" hardware, but I have heard bad things about the GPS and camera in the Nexus, which, combined, are about 80% of the reason I use a smart phone. I'm going to give the GS3 a shot and just hope that the wonderful folks at Cyanogen will work their magic.