The definition of illness is one of the perennials in the philosophy of medicine. Robert Freitas has a nice list in the first chapter of Nanomedicine ( http://www.nanomedicine.com/NMI/1.2.2.htm ) which is by no means exhaustive.

In practice, the typical "down-on-the-surgery-floor" approach is to judge whether a condition impairs "normal functioning". This is relative to everyday life and the kind of life the patient tries to live - and of course contains a lot of subjective judgements. Another good rule of thumb is that illness impairs the flexibility of someone - they have fewer possibilities.

Personally I prefer Freitas volitional model, where we give strong weight to the desires and goals of the patient. If I want to fly and could somehow be cured of weight, then that should be allowed. However, seeing medical interventions as allowed is not the same as claiming they have to be supported by everybody else (positive and negative rights and all that). There is much truth in saying that illness is what a society thinks we should be altruistic and pay for others, while health improvements beyond that tend to be up to the individual.

The problem is that altruism pool is limited (and quite possibly due to murky evolutionary psychology - consider Robin's "Showing that you care" paper) and shared resources limited, while the space of possible medical interventions is growing and human wants of course nearly unbounded. Hence there is a constant struggle for stakeholders to bring their conditions into the realm of altruism and obligatory treatment.

The problem is that we currently also roughly identify the category of illness treatments with allowable treatments (with some exceptions like preventative medicine, cosmetic surgery etc.) and the non-illness treatments as not allowed (doping, enhancement). This might be a reaction to rein in the costs and illness category, but also concerns that non-altruist medicine would be socially bad. I have strong suspicions this is misguided and actually decreases human happiness.

In the end, the goal of medicine should always be human flourishing, not health. Health is instrumental for living a good life, but what kind of health is needed depends very much on individual life projects. I believe that in the future we are going to see much more of a health pluralism.