Youtube – How to force YouTube to always use the arrow keys for playback seeking

Right-click on the video and select Stop downloading. To start it up again click the pause/play button.

Another way is to move the slider all the way to the end or near end of the video timeline. That way only the few remaining seconds left to play will end up streaming down to your computer and when it finishes, that's it. No more buffering or loading.

Firefox add-on SmartVideo for YouTube will do just that. You can even tell it to either start the video when you want. You can just open the link in a new tab, and just come back to it when you're ready to watch the vid. It has other useful options as well.

Edit: SmartVideo can be a bit buggy. As to 720p videos being buffered, I think it's because all videos in 720p (or higher? not sure) are already on WebM format. It's my default setting. It's the fastest video setting to load, faster than a 480p flv file.

Edit2: SmartVideo is also available for Google Chrome: https://chrome.google.com/webstore/detail/smartvideo-for-youtube/lnkdbjbjpnpjeciipoaflmpcddinpjjp?hl=en