Use a split button - main button downloads highest quality
This is just a passing suggestion, you're welcome to discard it if it's too complex. Personally, I only ever download the highest quality version of a video. It would be useful for me if the download button was split in to a main button and the downward arrow to see the rest of the options. The main button would then be a one click way to download the highest quality version rather than the current 2 click operation.
I use the script daily, so this change would be highly beneficial for me. It would be good to get an idea of whether others also use it in a similar way I imagine. If it turns out that I'm in the minority then this wouldn't be a sensible change.
This is a good idea. I'll see what I can do about it but it all depends on whether I can get the CSS to integrate with YouTube's layout.