So I've been playing around with video a bit more lately. I have two different cameras that record natively at 1920x1080 (1080p). I can get my videos into iMovie just fine, but one thing that always annoyed me is that when I went to share a move I created with iMovie, the default options were pretty pitiful.
The largest option for instance, is 960x540. If you export that, Youtube doesn't even show it is HD video. Hm.
Well, it turns out you can export with Quicktime in addition to the default export options. If you know what to choose, you can get good high quality HD video.
Choose Share, the "Export Using Quicktime". I'm saving my file types as quicktime mov files, which seems to work pretty well. You then have an option button by the video type and that brings up a dialog showing you the type of encoding for video and audio. H.264 video is pretty standard. Here is the kicker. By default, when I chose to export the movie, quicktime selected a very low bitrate which made for very ugly video. So you click on the video button and up comes the dialog as shown above. Choose to restrict the bitrate to some large number. 4000 to 9000 is plenty. You can experiment with short videos to see what you like best.
After that, export away. It takes a bit longer to export an HD video. Worth it though! Enjoy.