I'm biased because I like the whole Yanapay project, but here is what is special about the restaurant: it's designed like a huge tree house for kids, it's comfortable,and staff are warm and excellent. The menu features an extensive selection of delicious fresh foods, presentation is excellent, and there is a great S/15-20 menu that gets you an appetizer, main, dessert and a cocktail. The fact that all of the money goes to a sustainable project working (and succeeding) at creating "another way of life" for Cusco's children makes it even more delicious.
The restaurant is taken over by volunteers on Tuesday (which is bingo/quiz night) and Friday (which is the grand buffet). If you don't want to participate I suggest you eat upstairs on those nights, but the best thing to do would be to join the volunteers! People eat talk and play games together around a big table, and it is without a doubt the best way to meet cool new people in Cusco. By the end of the night you'll be convinced to stay and volunteer at the school!