Psychology is more an art form than a science. There is no "Theory of Everything" from which one can derive all mental health phenomena and make falsifiable predictions. Still, as far as personality disorders are concerned, it is easy to discern common features. Most personality disorders share a set of symptoms (as reported by the patient) and signs (as observed by the mental health practitioner).
Patients suffering from personality disorders have these things in common:

They are persistent, relentless, stubborn, and insistent (except those suffering from the Schizoid or the Avoidant Personality Disorders).

They feel entitled to - and vociferously demand - preferential treatment and privileged access to resources and personnel. They often complain about multiple symptoms. They get involved in "power plays" with authority figures (such as physicians, therapists, nurses, social workers, bosses, and bureaucrats) and rarely obey instructions or observe rules of conduct and procedure.

They hold themselves to be superior to others or, at the very least, unique. Many personality disorders involve an inflated self-perception and grandiosity. Such subjects are incapable of empathy (the ability to appreciate and respect the needs and wishes of other people). In therapy or medical treatment, they alienate the physician or therapist by treating her as inferior to them.

Patients with personality disorders are self-centered, self-preoccupied, repetitive, and, thus, boring.

Subjects with personality disorders seek to manipulate and exploit others. They trust no one and have a diminished capacity to love or intimately share because they do not trust or love themselves. They are socially maladaptive and emotionally unstable.

No one knows whether personality disorders are the tragic outcomes of nature or the sad follow-up to a lack of nurture by the patient's environment.

Generally speaking, though, most personality disorders start out in childhood and early adolescence as mere problems in personal development. Exacerbated by repeated abuse and rejection, they then become full-fledged dysfunctions. Personality disorders are rigid and enduring patterns of traits, emotions, and cognitions. In other words, they rarely "evolve" and are stable and all-pervasive, not episodic. By 'all-pervasive", I mean to say that they affect every area in the patient's life: his career, his interpersonal relationships, his social functioning.

Personality disorders cause unhappiness and are usually comorbid with mood and anxiety disorders. Most patients are ego-dystonic (except narcissists and psychopaths). They dislike and resent who they are, how they behave, and the pernicious and destructive effects they have on their nearest and dearest. Still, personality disorders are defense mechanisms writ large. Thus, few patients with personality disorders are truly self-aware or capable of life transforming introspective insights.

Patients with personality disorder typically suffer from a host of other psychiatric problems (example: depressive illnesses, or obsessions-compulsions). They are worn-out by the need to reign in their self-destructive and self-defeating impulses.

Patients with personality disorders have alloplastic defenses and an external locus of control. In other words: rather than accept responsibility for the consequences of their actions, they tend to blame other people or the outside world for their misfortune, failures, and circumstances. Consequently, they fall prey to paranoid persecutory delusions and anxieties. When stressed, they try to preempt (real or imaginary) threats by changing the rules of the game, introducing new variables, or by trying to manipulate their environment to conform to their needs. They regard everyone and everything as mere instruments of gratification.

Patients with Cluster B personality disorders (Narcissistic, Antisocial, Borderline, and Histrionic) are mostly ego-syntonic, even though they are faced with formidable character and behavioral deficits, emotional deficiencies and lability, and overwhelmingly wasted lives and squandered potentials. Such patients do not, on the whole, find their personality traits or behavior objectionable, unacceptable, disagreeable, or alien to their selves.

There is a clear distinction between patients with personality-disorders and patients with psychoses (schizophrenia-paranoia and the like). As opposed to the latter, the former have no hallucinations, delusions or thought disorders. At the extreme, subjects who suffer from the Borderline Personality Disorder experience brief psychotic "microepisodes", mostly during treatment. Patients with personality disorders are also fully oriented, with clear senses (sensorium), good memory and a satisfactory general fund of knowledge.