If the youtube window loses focus while in fullscreen, for whatever reason, it reverts back to small web size. This is a PITA for people with multi-monitors who would want to use one screen as a webtv.

So far, workarounds involve hacking the assembly code of the particular flashVM executable (won't work if you update). Another option is to use a flash video downloader and just view your videos offline.

Focus-lockable fullscreen is a must-have feature of webtv.