Our YouTube videos use the YouTube default quality settings, which seems to be based on a number of things, including your connection speed and the size of the video. If you want to watch a video in higher quality, you can try to adjust the resolution of that video using the quality selector in the lower right corner of the video. The lower the number, the lower the quality, but the faster it'll load. Not all of our videos were shot in High-Def, in which case those videos will not have all of the settings in the screenshot below.
If you always want the highest quality, click the 'settings' option, then on the subsequent screen you can setup your playback options to default to HD.