I remembered this little aspect of writing just a moment ago while commenting on someone else's blog and thought that I'd share this with you.
To me, one of the most important aspects of a story is your character's body language; the things they say simply with their body. I'll give you an example.
"Oh yes; you're very cute," Nicole said, smiling broadly.
"Oh yes; you're very cute," Nicole said, rolling her eyes.
The words said did not change in either example and there was not description as to the character's tone when she spoke. However, simply from the body language described at the end of each post, you can tell that, in the first example, Nicole was being genuine, if not polite, while in the second example she was being rude and sarcastic.
And of course, giving tone to dialogue isn't the only use for body language. It can also relate the reactions of other characters. If someone is talking and you simply say that your character is listening, then the reader doesn't know their reaction to what is being said. However, if you describe them as having their head resting lazily on their hand or their eyes being averted out the window rather than at the speaker, the reader knows that your character is bored. Alternately, if the character is leaning forward with a light smile and bright eyes, it gives the impression that your character is interested and listening intently.
So that's my two cents on character body language. In my opinion, the things your character is doing with their body is as important as what they say as far as voicing their thoughts and moods in the book, and personally, I believe that they should be described to the fullest.