It depends on how one chooses to look at the question. Is it rehab from the perspective of the sufferer of the brain injury or his caregivers thereafter? Since I work as a speech language pathologist with this population, I'm going to say there's 3 things that enhance the recovery: a passionate therapy team, an equal balance of give and take between client and therapist, and mutual motivation and respect for any progress (big or small; gradual or progressive) being made.
Even when I worked in long term care, the older adults needed a good dose of "business versus pleasure" in their therapy regime. What I mean by that is always making sure they feel a connection to the tasks you are engaging them in. It's easy as a therapist to only focus on dishing out exercises, worksheets, etc., but what about the guidance and insight one can offer through discussion of these same exercises, worksheets, etc.? I have seen the most success in my patients when they find themselves reflected back when looking me in the eye.
I hope this makes sense!