There is a certain freedom of feeling in outdoor portraiture. It's an infectious feeling; the subject experiences it when posing, the viewer senses it when looking at the photograph, and the photographer feels it when making the portrait. It might well be that outdoor portraits seem the most natural to people even though, in fact, a lot of outdoor poses are the most contrived of all. For example, have you ever actually seen a young woman crawling up from the surf at the beach as they do in the Sports Illustrated swimsuit edition? Of course not. But it seems natural.
A great deal of fashion work, a growing amount of private portraiture work, and a lot of commercial portrait work takes place outside. I've done portrait-style work at company functions from tropical paradises to wilderness retreats, and very often I take a client outside to a park or a field to create a portrait.
The joy of working outside is the same as the drawback: although you can explore a wide variety of backdrops and props from the natural world, you have a limited control over how you can use or direct them. You can't, for example, just move the signpost that is in the way or make the wind stop while you expose your shot.
The best advice about working outdoors that I can think of seems the most obvious: think big. Take advantage of the energy inherent in the atmosphere; make your model ...