What it does:
- Blocks unnecessary elements using the display property.
- Sets masthead and page background colors to grey (compare to default page background color).
- Removes boxes surrounding the video description and title.

What it doesn't do:
- It doesn't emulate YouTube feather, the only similarity is that it trims the fat off.
- It doesn't necessarily prevent the elements from loading, so your page loads will still probably take just as long.

If you'd like some page element back:
1.) Go to Firefox > Add-Ons > User Styles > Edit
2.) Read the comments /* that look like this */ and find the element you want back.
3.) Delete the line or group of lines directly below that comment.
4.) Click "Save"
5.) Reload affected page.

See: http://freestyler.ws/style/130765/youtube-minimal-grey
if you'd like something in the same vein, but a bit more usable.