> So is it simply a matter of degree, and the "disorder" label just gets applied when the traits get strong enough for others to comment on?
Yes, it is the degree to which traits manifest, but it's not simply a matter of others commenting on them. Typically the degree to which these disorders manifest needs to interfere with living a normal, productive life. That can mean something different to different people, but it's safe to say that if your narcissistic rage prevents you from getting/keeping a job, that your case is likely a certifiable disorder.
The trouble with personality disorders (which are pretty interesting, btw) is that if you ask 10 mental health professionals you will probably get at least 8 differing opinions (these numbers are fabricated on the spot, the point is that there is a lot of variation). Combined with the fact that personality disorders are largely untreatable the whole thing becomes a spaghetti mess.
Studying personality disorders is really interesting and can offer a lot of insight into normal social interactions but I would caution people about second-year syndrome [0]. Keep in mind that, as someone else pointed out, these disorders are extreme cases of normal behavior. Don't get all freaked out when reading about mental disorders because you think you exhibit signs of schizophrenia, histrionic or narcissistic disorders. You do, we all do, it's normal - just not in the extremes that those with actual disorders exhibit.
Disclaimer: I am not a mental health professional, though I do work with them and their clients daily. I also minored in Psychology, which is valuable for a C.S. major when it comes to social interaction or marketing your skills/ideas internally.
[0] http://en.wikipedia.org/wiki/Medical_students_disease