perspective (or the face you don't recognize with that big nose in the middle :-)
The folks I know who do portraits on a regular basis swear that 100mm to 135mm (35 mm cameras) lenses are the best for such work--just to avoid the problem you mention.
These lenses also mean that you can get detailed shots without getting too close to the subject. "Remember the I35W bridge--who needs terrorists when there are Republicans"
For portraits - in the sense of a depiction of a person - that include the environment you can use much wider lenses without making the person look too scary. It's all a matter of how much of the frame they're filling.
