YouTube often has problems displaying 16:9 aspect ratio wide screen videos so that if on editing and playback (especially if using Windows Movie Maker or the new Windows Live Movie Maker) videos look fine then it's likely an issue with YouTube and not necessarily the video. To fix this problem YouTube provide a number of 'tags' that can be added to a videos 'properties' during or after upload.
Tag: yt:crop=16:9
(zooms in on the 16:9 area, removes windowboxing)
Tag: yt:stretch=16:9
(fixes anamorphic content by scaling to 16:9)
Tag: yt:stretch=4:3
(fixes 720x480 content that is the wrong aspect ratio by scaling to 4:3)
Tag: yt:quality=high
(default to a high quality stream, depending on availability)
