Now consider that you want such a character to be involved in collisions, for example, preventing the character from passing through a wall. The collision system needs to support skinned, continuous level of detail meshes. You might even envision such a character being partially controlled by key frames and partially controlled by inverse kinematics. Another example is integration of game physics with collision detection. Not always an easy match.
Integration is key to a successful commercial engine.
I believe that general visibility determination on current machines is still not possible in real-time. Just as with collision detection, the visibility system should make every attempt to use specific information about the world. The design of the world itself needs to take into account what the plans are for use of the world. If you build a world such that the camera can be in a location for which a dozen alpha-blended trees are stacked up along the line of sight, you are only asking for performance troubles. My experience in dealing with some NetImmerse customers is that artists are generally unsupervised about such issues.