Defining life with the amount of knowledge that humans have, currently, may be trivial. As humans know it now, our life might represent all life perfectly or it could only represent the smallest sub-section. So, before the definition of life can be determined there needs to be more data collected about what life really is and how it affects other beings.
Humans have enough trouble defining what our own life is that it almost seems impossible with the given information that we would be able to define what marks something else as living. So far, for life on earth, it seems as though life needs some sort of structure to it and that it needs some organization. It also appears that in order to be life on Earth, there needs to be evolution. This is a very common theory and it seems logical, but it might not apply to all life forms and there is no way of really knowing given our current knowledge.
Other beings, as far as we know, could have a completely separate system for life. All of the possibilities that their systems could be are unimaginable. Yet still, they could be very similar to human life. With the current information that we have there is really no way of knowing how to define life for all beings.
If there had to be a definition for life, there should be one for each being. Humans should only evaluate what gives them life, since all that they know is Earthly life. If there is ever something that could resemble life, a new definition should be made for this new life form. This will take all of the guesswork out of defining universal life.
I also have trouble imagining that life on earth would have the same root cause as life may be on other worlds. The structure of organized building blocks like cells seem to be very complex and might not be the same for extraterrestrial life. I don't agree with the definition for each being though because that wouldn't help us define life, which is useful for identifying life on other planets.
I agree with you when you mention that life cannot be defined currently because there is no other samples that we know of. Without discovering life somewhere else in the cosmos, we have no idea what other life has in store. I also find it interesting that we may be a "sub-section", an outlier. For all we know, we may be a planet with an extremely unique way of life.
However, I disagree with what you said when you mentioned defining what it means to be a member of a species. I believe that does not really accomplish much and that there would be no true definition due to Darwinian Evolution. Also, life is already divided up into subsections like: reptiles, birds, mammals, amphibians, etc.