Did a bit more testing with the LG ST 600 and so far it plays everything I have tossed at it, accept one movie that I think has some errors as most of my other devices have issues with it.
I was able to play 780p mov, mp4, avi and mkv files both directly over wifi/share and DLNA.
I downloaded a 1080p x264 trailer and was able to play it over wifi cleanly (may depend on bitrate)
This box seems very capable, my only issue is the UI which frankly sucks.