Frequent and occasional users of Youtube.com would have noticed a big/wide change today when you tried viewing a video on their website.
Having introduced the widescreen player for all their videos on 25th November, Youtube has gone one step further by now introducing a watch in HD option (for non-HD, the watch in high quality option remains). This option will come into play if your videos are uploaded in 1280×720 resolution. This option was actually working for a few months already but you had to use a url hack by adding &fmt=22 to the end of a video url. So now, Youtube has truly gone high definition with a widescreen player and the ability to watch HD by default.
Watching HD on Youtube is awesome. But it also means that it would be laggy for those who have computers with lower specifications. To get videos with high quality back (less lag also), add &fmt=18 to the end of the video url. It will also take longer to upload a video in future because of the bigger filesize.