Maybe it's obvious, but I see people, top professionals overlooking this question, as if it's just about gut feeling and experience.
For a few years now I have been taking part in therapist working groups in various forums. In these, a case is for consultation in confidence by a therapist, and a group of colleagues provide additional insight to that of the therapist. It's a great thing to do for many reasons that are not the issue of this post.
Often, a patient is described, for example, let's imagine a patient that is dealing with a fear of flight. The therapists think, analyze the meaning of this fear, and reach a rich description of how it probably feels like to be the patient, and what deep meaning every common thing bears for the patient. For example, the fear is related to that patient's need of control. This is very easy for therapists to achieve, as it requires a simple summation of their most creative associations.
Then sometimes comes a question: so what to do about it? We understand that the patient wishes for, or even needs control - do we give him (could be her, sure) some sort of control, or do we make him give up control? We must see that both are possible, and both could be good. Challenging a patient out of his comfort zone is good. Rushing a patient out of his safety zone is bad. Many discuss these as matters of faith and personal taste. That makes them right about half of their decisions, and blind about the factor that makes them right or wrong.
So, to my view, and I am very surprised at this being an overlooked issue, the issue is ego powers. I see a person's mental powers as a sinusoidal wave, with ups and downs over time. A person could meet a difficulty when he's down, and that would weaken and deplete his ego (mental capacities) even further, or, he can meet a difficulty when he's strong, and that would forge him even stronger. The same difficulty could ruin or build. Challenging a patient should then be considered according to his mental capacity to build from that as a challenge and not weaken from it as a handicapping intervention.
People need challenges, to grow, to be able to experience the world, with its excitement and hardships. Being a nice and facilitating a way around an obstacle isn't serving the patients' needs. Help could be handicapping too, if it does not acknowledge the capacity of the patient to take on challenge.
This changes over time for every person. In the insightful description of Stephan Mitchell of how kids grow their capacity to deal with the world through their parents' mediation, he describes a child that ask for juice at the age of two. The parent applause the child, and provides the juice. At the age of 3 that child, a verbose and industrious creature already, asks for juice, and is suddenly asked to say please or even get it himself (or herself, of course). We keep growing, and our expectations of ourselves need to keep advancing a bit ahead of where we are. Just a bit.
My metaphor for this is of the Israeli MANGAL. The charcoal grill. We light it with fire, then we fan the flames with wind. If you fan it too soon, you turn the fire out, if you fan it too late, you lose the fire's grip on the charcoal. To fan it in time is the work of the master MANGALIST. The same is for challenge in therapy.
PS - I apologize for either writing the obvious, or saying that it's obvious.
PS (10.9.2012): I think this makes more sense talking about agency. And I also think Winnicott prethought of it.