Product comments allow you to share your thoughts about a product on the product page for the world to see. They also allow other people using Supermarket to view your favorites page (unless you choose to keep your favorites private).
Adding a product to your favorites lets you keep track of the products you love and might want to go back and buy. Other people using Supermarket can look at your favorites and see what impeccable taste you have (unless you choose to keep your favorites page private).
Sure, if you have a blog or MySpace page or some other presence on the internet you can add a link to your favorites page. Just copy and paste the url (e.g. supermarkethq.com/users/987654321 or supermarkethq.com/users/username)