YouTube come with clever way to reduce the downloading of the highest quality 1080p content:
Many videos (like this one: http://www.youtube.com/watch?v=GbVxvITmwIc or http://www.youtube.com/watch?v=bVMwhOYqA6U ) have only the 720p videos WITH sound. 1080p videos are w/o sound.
Downloadable for example there: http://en.savefrom.net/
Now it is possible to combine the audio from 720p videos with the 1080p video in VirtualDub, but how can be YTD told to download the 1080p videos? I see no way ATM. Maybe it could be changed...?
YTD only downloads complete stuff. Partial downloads are not supported.