Fundamentally, Dorian is narcissistic, vain, and selfish. He is obsessed with beauty, and sensual experience to the point that he has no balance in his life. He is so focused on fulfilling his sensual desires that he loses touch with reality. While these characteristics do intensify as the novel progresses they are evident even when we first meet him at age 20.
The first direct sign of Dorian's downfall is his unsympathetic response to Sybil's suicide. The fleeting nature of his love/lust for her suggests that he is imbalanced. Furthermore, this sign, if not initially recognized by the reader, is confirmed when it represented as the first change in the portrait.
As his obsession with beauty, vanity, and himself intensifies, Dorian becomes more mentally unstable, eventually resulting in him killing Basil. From this point on, the reader can see Dorian's descent into madness is inevitably going to result in his demise.
Ultimately, his flaw was his impressionability. I think it could be interpreted here that Wilde was mocking the Victorian belief at the time that children should remain as innocent as possible and know as little about the cruelties and "evils" of the real world as they can.
Dorian could be seen as a literal manifestation of that sort of upbringing, and because he was so naive in the beginning and remained so impressionable, it was easy for him to become narcissistic and vain without any full realization of his actions. Because he was so "innocent", it was that much easier for him be swallowed up by evil and insanity.
Even in the end, I don't think Dorian ever became as cynical as Lord Henry. (it's a while since I read the book though) He was still just as impressionable, corrupted as he was. Everything he became was born from outside influences, never really from himself. Narcissism and selfishness were sort of byproducts.