Here is how I make the viewing experience on YouTube that little bit more pleasant (using Safari).
Firstly, I opt-in to the YouTube HTML5 trial which replaces Flash videos with H.264.
Next, I install the [A Cleaner Internet][2] Safari extension which declutters a lot of the site.
Finally, I apply a stylesheet which removes the "You need to upgrade your Adobe Flash Player" warning that appears when flash is not installed (user.css). If a video requires flash then I open it in Google Chrome using the "Open With..." part of the Develop menu.