I suppose I can agree that being the subject of the life has some value, in that we do not really need to deal with the ethics of how to treat rocks and dirt. I think, however, that the focus should fall more on certain relevant characteristics of life. Sentience, for instance, is a relevant characteristic of life, whereas the ability to respond to a stimulus is less relevant, given that it doesn't really require much in the way of biological complexity in order to do so. I don't think it would suffice to choose more complex relevant characteristics given the diversity of the human experience and ability. I imagine that we could draft some more relevant characteristics.
I suppose that the problem, after you choose the relevant characteristics, is deciding what kind of treatment is appropriate for each degree/extend of the relevant characteristic. We need to be ever aware that humans can fall very low and very high (since they are currently the point to which all others are scaled) on the degree to which they hold any characteristic.