Shooting the Kids - » Point & Shoot Depth of Field - Part I:

Depth of field and tight-cropping are, I believe, the best way of producing a sense of intimacy in an image. The blurring of a background (or a foreground) implies focus (in all sense of the word) on the subject. A tight crop gets the same sort of result, but by excluding other content.

Each has a different feel to it, and when to use each one is a matter of personal choice for the image you're taking.