Never write about well-adjusted people. It is a lesson I've learned in my many years of reading fiction and my few years of writing it seriously. No one cares about well-adjusted people; Aristotle was onto something with his depiction of the tragic protagonist as necessitating a flaw. It is the imperative of an interesting story that the protagonist have a weakness. The difficulty faced by Aristotle's model is that it ultimately fails in two important ways.
Qualities: Aristotle's Poetics insists that a tragedy must feature a strong protagonist of good moral character - someone who can be looked up to and whose strengths are truly heroic in nature. Such a hero is to have a single tragic flaw which eventually becomes his undoing. This is an interesting character model, but it is ultimately not the most compelling character one can work from. The tragic anti-hero (to completely disregard the meaning of the word) is by far more interesting; a protagonist riddled with flaws, lacking most of the qualities one might associate with a protagonist and who manages in spite of his flaws to achieve a kind of virtue. This character model brings us to the second way Aristotle's model fails.
Virtue: Aristotle's tragic hero is virtuous because of his strengths and he fails because of his single flaw. What is important in this model is that his flaw betrays him in the pursuit of a virtuous aim. While this might be interesting at first, it appears formulaic and tends to encourage monotony in the tragic medium. The far more interesting character model, the anti-hero mentioned above, has the distinction of failing, not in spite of his virtue (or quest for virtue), but because of it. That he exists at all when he is encountered is a testament to the strange stability he has established between his flawed self and the society around him. The quest he is forced into, or chooses, interrupts that stability and his failure stems from that interruption, not from his flaws. In this sense, the character tends to be more tragic than the traditional tragic hero, in that the virtuous end he seeks, itself, is his undoing.
To suggest, as it seems I have, that this is the only interesting way to write a tragedy is inaccurate; as with Aristotle's model, if it is followed religiously, it would tend to monotony and formulaicity. The only argument I've made here is that a more dysfunctional protagonist leads to a far more interesting story, and that dysfunction in general tends to draw more interest than its opposite; proper tragic fiction should keep this in mind, while keeping an eye to originality, innovation and surprise - without surprise, the exercise of reading fiction is essentially futile, as the outcome becomes obvious well in advance of the denouement and the reader tends to close the book before he or she has reached that point.
Screwed up and original; without those two factors, fiction is boring.