Probably the most consistently argued about at an OFL game is the tag. This is why a legal tag is important.
Tags must be two hands to the body. Not fingertips; not to a piece of shirt or the ball. Two hands to the body. Arms do not count.
The defender -- once he's made the tag -- should then loudly say "I Got Ya!" or "Down!" or something. Yelling "I Got Ya!" before actually tagging a ball-carrier is not cool (yes, it happens a lot..) The play stops when you announce "Got Ya!" or "Down!" Therefore if you say it before you actually get the ball-carrier, you're cheating. Illegal (phantom) tagging is a 15-pace penalty from the spot of the foul.
A offensive player is considered "down" when a defensive player tags with ball-carrier with two hands fully contacting the body (one hand if the player is on the ground). Touching only clothing (i.e. a small bit of T-shirt or the scrimmage vest), or tagging with fingertips, does not constitute a successful tag. Additionally, the ball does not count as part of the body.
Grabbing a ball carrier's clothes to stop him/her is a penalty. Tackling is not allowed.
Notes:
As such, defensive players should be very sure they have applied a legal tag before yelling "Gotcha". A false tag is a penalty - it stops play dead, and everyone gets mad and the whole thing generally sucks.
Other note: If the quarterback is directly under center (for a direct snap), defenders cannot reach across the line of scrimmage for a tag, even on a blitz. The defender must have his feet across the LOS before a tag is allowed.