perhaps the single most comfortable thing a person can wear. they are very simple, staying up around your waist by an elastic or a drawstring.
sweat pants are warm, since the only place that cold air could drift in is cut off by elastic that hugs your ankles. the insides of these pants is fuzzy and cozy, and it's like jumping into a puddle of hot chocolate.
the one drawback is that sweat pants are only acceptable-looking on some sort of jock. the rest of us stubby-legged people just look dumpy.