Things I look for in professional images:
They always say a book is only as strong as your weakest image. So if you have an image in your book just to have more images, its a mistake if its a weak image. 
Models with many different expressions; its very easy to tell a professional model apart from a girl who just wants to look pretty in all her photos. I want to see strong expressions in happy to fierce, to calm and or seductive. There is a difference in faking these expressions, or to actually make the people looking at the photograph feel like your feeling those emotions.

Polaroids/Snapshots:
Fullshot, models porportions and curves
Face, features and symmetry, smiles if for a commercial shoot.

They are simple to take. Can take them with a point and shoot camera with no lighting other than the natural lighting coming in from the windows, better to take with no flash.

Great polaroids:
http://www.chanelrene.com/polaroids