Benoire:
Sorry, yes 4.5mbps can be HD as the resolution is what defines HD/SD not bitrate, however I wouldn't consider 4.5mbps anywhere near the correct bitrate for 720p let alone 1080i. I've got an mkv with a resolution of 1920x800 that is 5.3mbps and I can see the blockyness in the dark areas and greys even with high level passing done on the rip...
Suspect your 1920*800 mkv is a 1080p movie that has had the black borders chopped off, hence the lower vertical resolution, thus the bit rate is a bit low. However with the right encoding anything above 3500kbps can easily be 720p, go onto any pirate website and you'll frequently find 720p mkvs encoded at bit rates lower than that.
You have to be realistic. This service provides video at a higher bit rate than any other commercial website in NZ (TVNZ, iSky etc). If you look hard enough you will see artefacts in every encode, its a fact of compression.
The new video suffers from the same poor encoding as the last one, hopefully they fix it. Clearly they are listening though as this is the third preview clip they've uploaded.