The hardest part, in my opinion, is dealing with many different types of groups on the paintball field. A group of guys (age 25-35) would probably be your best scinerio as they have usually shot a real firearm and understand the importance of goggle safety and gun handling. This would be, generally, an easy day of reffing. A 12 year old birthday party might be you worst nightmare, as they might be unpredictable with goggle safety and barrel socks.
Also, you need to know when to be friendly (if the group is well behaved) and when to show authority (if the group does not listen to rules well). If you come off as a pushover, the group is going to pick up on that.
Bottomn line - you're protecting people's eyeballs, make sure they leave with just as many as they showed up with.