FYI, you can replace "watch?v=12345" to "/v/12345" to view videos without logging in (works for age-restricted content as well). As a bonus, the video fills the browser screen, and removes all the annoying "related" content, comments, etc.
Or if you're on Linux, you can use youtube-dl + mplayer to get a high-quality hardware-accelerated glitch-free viewing experience, instead of the RealPlayer-esque experience Flash provides.
Also try youtubedown in place of youtube-dl - I think it works with a bigger percentage of videos.