Nice article by Darren Rowse on the effect of different focal lengths on photo composition, especially the images used to illustrate his point. Zoom isn’t just for bringing a distant object close – varying zoom lets you vary your distance from the subject, which has a huge impact on the surrounding space and the background. Rowse doesn’t mention bokeh, the term applied to interesting, patterned background, but you can create bokeh by moving away from a close subject and zooming in. (The distance between the subject and the background is also significant.)
How to Use Your Zoom Lens as a Compositional Aid by Darren Rowse, Digital Photography School
You can see this principle illustrated really nicely in the images [at the link]. While the model takes up much the same amount of space in each of the shots – the five different focal lengths product quite different compositions. None are particularly ‘bad’ photos – but each produces very different results.