The Theatrical release suffered through being long but disjointed, with fractured and at times jarring story-telling.
The restored scenes in this extended cut make for a more satisfying and coherent narrative, and bring us closer to Tolkien's original work.
Stand out extra scenes are:
Saruman's last stand; a conflation of the original, but a great scene, faithful to the themes and spirit of the scene in the book. This gives the closure the Saruman story deserved.

Aragon draws the eye of Sauron: this serves to partly explain Denethor's madness, but not as specifically as the book does.

The Mouth of Sauron: A truly revolting and effective creation.
The Mimis Tirith battle scenes are also extended, and we get to see that revolting Orc Captain get his deserts. He just vanishes from the Theatrical version.

As for the documentary features, there's a wonderful, inspiring documentary on Tolkien and the book, intelligent and illuminating.
The next feature is also good, looking at some of thr processes involved in filming the unfilmable. There's a fascinating look at a scene that never was, but we see here storyboarded, where Sauron appears to Aragon in his original Angelic form, before taking on his 'Dark Lord' form to do battle.
There's more, enough detail to satisfy the most obsessive fan. This is a good value dvd package, an essential cut of what until now has been a very flawed release.