The concept that a character in a tragedy must have a "tragic flaw" is based on a popular misreading of Aristotle. There are a few different structural concepts of tragedy conflated in this misapprehension. First, a tragic character must have some measure of greatness. Second, the character is not perfect, and some point makes an irrevocable bad choice. The bad choice is a like a spear that once cast cannot be taken back ("hamartein" means "to miss"). Finally, often the character is placed in a situation where all possible outcomes are bad.
For Romeo and Juliet, as mentioned above, the basic character flaws are impulsiveness (they are teenagers!) and tunnel vision. They think of nothing but a desire to consummate their relationship immediately, and in their self-centered visions harm their families and eventually themselves. Had they been more mature, their love could have enabled them to work slowly and steadily for reconciliation between their families.
Impulsiveness--They're not the only ones (Mercutio, Capulet, and others are ruled by their emotions), but they clearly don't think much before acting. What if Romeo had followed his head instead of his heart when he first saw Juliet? What if they both had considered the angles before killing themselves?
A flawed view of love--They think of love not as commitment that involves sacrifice and work along with pleasure and attraction, but just as a euphoria that they must seize and keep. Having a more mature idea of what love and marriage entail might have kept them alive--but then, we'd never have heard of their epic affair. Prudence doesn't make for good theatre.
Failure to communicate--They seem to think they can solve all their own problems. With the exception of the Nurse and Friar Lawrence, they don't involve anyone else in their own schemes--not even each other.