It's probably worth exersing alittle skepticism towards DSM mental health categories like schizophrenia, and schizoid-type personality disorder. Many psychiatrist (from the mid-20th century onwards) have infact staunchly rejected such categories as pinpointing no measurable physiological characteristics, and being chronically ineffective in aiding people back to 'health'. People like Szasz and R.D.Laing have probably been the most vocal critics against psychiatric categories - preferring instead to refer to 'problems in living' and addressing those, rather than obsessively inventing more and more pseudo-ontological categories.
Ian Hacking went as far as to say that psychiatric categories are potentially counter-productive because they create a 'looping effect' whereby people start 'acting out' the traits of their ascribed mental disorder.
Possibly all irrelevant thou, since most people here are using the term 'mental illness' in the tricksy sociological sense - stigmatising* an undesirable social group for the sole purpose of discrediting their experience and views.
*Erving Goffman has written some interesting stuff on this topic.
There are many problems with the DSM - we are up to number 5, I believe. One issue you did not mention is the implicit - and philosophically weak - notions of cause that are implicit in the way individuals get pathologized. IOW issues that might be much better looked at as public health issues or (within the range of) normal responses to damaging environments are isolated from their environments and the individuals are seen as 'having' a mental illness. Wonderfully, and yes I am quite sure coincidentally, these illnesses are hazy and create enormous customer bases for the pharmaceutical companies. A couple of recent books that partially come in from this angle is
Making Us Crazy: DSM: The Psychiatric Bible and the Creation of Mental Disorders
and
The Selling of DSM: The Rhetoric of Science in Psychiatry (Social Problems and Social Issues) (Paperback)
a classic focusing most on one drug is
Talking Back to Prozac, Peter Breggin
There was also a wonderful, very funny (ultimately) book in which a woman psychologist laid out her proposal for a new diagnostic category, which she submitted to the DSM committee. This disorder basically described the psychiatric attitude. Her point was that a number of the categories in the DSM pathologize women and use a very specific group of primarily men's norms for social activity and emotional expression as the guide to making categories that end up being filled mainly with women.
Her proposal, presented exactly as other categories in the DSM with symptoms, differential diagnosis, etc., was expertly written and most people, frankly, would never have thought it out of place in the book, except perhaps the psychiatrists - a sociological category - both male and female who might catch on they fit the bill.
I wish I could remember the title of that book.