I think there's a lot of potential in this version of Watson, since she's a damaged character with some interesting textures that could be built on. I like the idea of Holmes recognizing a kindred spirit in her and trying to bring it out. It's not just Holmes lecturing Watson about his deductive methods, but actually nurturing the same gift in her, helping her discover and embrace that part of herself. It does feel a lot like the Doctor recruiting a new companion, admittedly, but in the context of Holmes and Watson it's an interesting twist on the dynamic.
It's true, though, that Liu's performance doesn't impress me yet. I haven't seen enough of her work to have a firm opinion of her overall abilities as an actress, but so far she's not bringing much to the part beyond those stunning eyes.