You can check out some professional demo reels here:

http://forums.cgsociety.org/forumdisplay.php?f=154

Just look for the threads with "Demo" in the title. (You may have to join in order to see the videos).

If you are planning to submit to a major studio, you might want to check their requirements for submission. They may want a certain format, a certain length, and they may ask you to demonstrate certain skills. For instance, BlueSky wants animators to be able to rig, if necessary.

Generally it's a good idea to make sure you clearly demonstrate a mastery of the 12 principles of animation. Weight is very important. (Weight is rather difficult).

Lip-synch is important. Don't expect every place you apply at to be using automated lip-synching software.

Acting is super important. It's important that the character seem like a thinking, feeling being.

If there's anything special that you know well, (Kung-fu, juggling, dancing, etc.) put that on there as well.

And in case anyone wants to see some more cool stuff done with A:M, you can find it here: http://makeashorterlink.com/?L5B722E6C