I think 'extreme' sport has nothing to do with what it is being done, but how it is portrayed.
It's nothing to do with injury really - I play rugby on weekends, definately higher injury rate than paintball (2 people broken bones, back injuries etc.) just this season. However it's not an extreme sport!
People take extreme sports to be different from the norm and usually associate them with things like you see in tourney ball. (Bright coloured clothing, strange headgear(!), shiny equipment)
I was riding my bike a while ago and came along a group of proper off road trail/mountain bikers, and was amazed that the clothes they wore looked exactly the same as paintball stuff, and the bikes looked the same style as paintball guns, and the helmets were styled the same as our masks.
Nobody really can define an extreme sport, I feel paintball is a bit extreme, because it is different, aggressive, and does hurt!