Aang was often depicted as being childish and playful, which led to great character development when he had to take things seriously. It also helped balance the show out, seeing as it could get pretty tense at times.
Ozai used an ice blast to scar Zuko. Since Aang is the last earthbender Appa would be the last badgermole after the Earth Kingdom was flooded. Instead of ash, snowfall is the first sign of a Water Tribe attack.