Interpersonal relationships are the heart of my favorite genre. Romantic relationships are simple and easily mapped and a lot of fun. But other levels of interpersonal relationship include families and friends and colleagues and people brushed by on the street and forgotten. Interactions with other people are the most visible form in stories from Romeo and Juliet to My Sister's Keeper to Twilight.
Intrapersonal relationships are how we see ourselves. It can take the form of akrasia or contemplation or Hamlet's famous soliloquy.
Environmental relationships are how we interact with our world. Is it snowing and cold and impossible to be outside in, or gorgeously sunny and warm? Is the atmosphere even breathable oxygen? Is our local world full of meth heads and drunks or puppies and white picket fences? Is the main character an eco-loving vegan or indifferently concrete-bound?
Obviously world-building plays an important part in the last, but it plays just as important a role in the other two relationships. With interpersonal relationships, it is a fairly clear-cut role: a romantic relationship might be proscribed or directed by anything from caste to location to politics. With intrapersonal relationships, the degree of introspection expected of an individual will vary wildly between cultures, which will inform that relationship.
. . . Oh, right, I was going to talk about something that wasn't world-building. We'll try that next time.