For example, we've already seen the part about Sean Astin cutting his foot, and the part when Elijah comments on the beautiful sunrise in New Zealand, stuff like that. But we haven't seen a lot of Miranda Otto and her role as Eowyn, and we haven't seen much in the way of the details behind Helm's Deep and what the cast went through to film it.
The goodies in this special are mostly the personal behind-the-scenes moments - Dom and Billy playing pool, Dom bungee diving to overcome his fear of heights, Orlando's desciption of the serious back injury he suffered a year before being cast as Legolas, Sean's directorial angle, etc. And the love the cast have for one another is much more tangible here than in other stuff we've seen.
OK - you can see the rest for yourself!