Hey flyn,
You've not discovered enough about the cart yet. There are more than two tabs by default - at least on the default_html5.
You simply need to add something into those areas for them to show up, and yes they only show on products where there is something in those fields. They are Tag and relations.
Tags are managed on the product description page - and are helpful in the search by adding further classifications to products beyond categories
The Relations are managed and found in the relations tab (across the top tabs) Field is Related Products - this is a great tool for cross-marketing, and it works well. Also when you add a relation into one product it automatically adds it into your other product (least in 1.1.9) Eg. Prod a add a relation to Prod b. Prod b will automatically add a relation to Prod a.
Very cool!
Now as to your videos and additional info - a couple of ways - you can add the video within the product description area it handles an I-frame well. You can add as much description on the block as you wish and I see some of the carts that look like an Amazon page. You can also purchase an extension called Product Features which adds additional tab and you can place your additional information in there. It also only shows the new tab when there is something in it.
Hope this helps you to understand more of the functionality of the cart.
Lee