Cook couscous according to package directions. Fluff with a fork.
Meanwhile, in large bowl combine tomatoes, cucumber, sweet pepper, chives, parsley, mint, and oregano. Fold in couscous.
In a small bowl whisk together the balsamic vinegar, olive oil, sugar, 1/2 teaspoon salt, and 1/4 teaspoon freshly ground black pepper. Pour over couscous mixture and toss to combine. Spoon into serving bowl or cover and chill up to 24 hours. To serve, top with feta cheese and walnuts. Makes 12 side-dish servings.