I think another reason why robotic limbs have a tendency to look unnatural is that often they'll move at a constant rate of speed - unlike an actual limbs that

I think another reason why robotic limbs have a tendency to look unnatural is that often they'll move at a constant rate of speed - unlike an actual limbs that accelerate, move at X speed, then decelerate. This is important for UI designers/developers to keep in mind because the same thing will make any animation look unnatural. Most UI frameworks have animation support for the penner easing equations and you should definitely use them. Its something that it is easy to tell that something "isn't right", but its hard to tell exactly what it is.