In relation with a recent Cochrane review, the paper discusses desirable features of a "humanoid" robot-therapist: 1) high mechanical compliance, 2) large range of force, 3) minimum assistance level, 4) soft haptic interaction for proprioceptive awareness, 5) adaptative assistance properties. It also proposes a framework for addressing optimal assistance and learning paradigms in view of a consensus in the community of rehabilitation engineers about shared principles and common standards.