There are some definite no's:
1. No fan has the right to yell negatively and then attach either a player's name or number to their comment.
2. Keep in mind that the reason that the Lord may have given you the loudest mouth is that he gave you the smallest brain. You may not be a genius, and if you scream the vote may be unanimous.
3. Never ever criticize or yell at small people such as volunteer ball girls, water girls and boys, etc. Fans that do that are not fans.
I tell my parents that I want them to yell every positive comment they can every game. They are to be the biggest cheerleader for the child, but never to criticize any other player on either team. They do a pretty good job of that.
Also not a good idea to approach a coach, official, or opponent after the game. It is amazing to me how many thoughtless parents being their biggest life problem to you immediately after a four hour soccer event without realizing or caring about the service you have provided to their children. Sleep on it and call me tomorrow if your world is going to collapse. I have never gone up to a parent and said anything other than "__ played great."
The only person who can yell "The Coach is an idiot!" is his current spouse. Ex-spouses are not allowed to do that.