Comedy is one of my favourite genres--be it literary, cinematic, theatrical. And the defining feature of comedy: comic characters have boundless energy to meet life's diverse obstacles.
Think of Laurel and Hardy's indefatigable attempts to move that piano up those flights of stairs. Think of Coyote's relentless pursuit of the ever-evasive Roadrunner. Or think of Didi and Gogo in Beckett's Waiting for Godot: only in a manner of speaking are the two tramps actually waiting for the no-show Godot; for the most part they are consistently active, questioning, exploring, resisting and wrestling with their peculiar situation. They are down, but not out. They are, in short, energetic, in spite of their circumstances.
On the other hand, tragedy is about ever-diminishing levels of energy. Tragic characters run out of steam and expire. This is true of Oedipus, of Hamlet, of Willy Loman. MacBeth might be the best example of this: he begins the drama as energetic and powerful, but his energy and power drain away as the drama progresses.
Another defining feature of comedy, one that's too-often overlooked: comedy and the laughter it creates aren't expressions of happiness. Why do people assume that comedians are cheerful, happy people just because they make others laugh? (As I write these words I'm thinking of Robin Williams, the actor-comedian who committed suicide earlier this week.) We too often think of comedy as all sunshine, but it's not. Comedy and laughter emerge out of darkness, but from that darkness they offer (sometimes too-thin) bulwarks against unhappiness, helplessness and fear. And so tragedy and comedy are a kind of tandem experience. As Beckett wrote in Godot: "The tears of the world are a constant quantity. For each one who begins to weep, somewhere else another stops. The same is true of the laugh."