Rulings

Shield counters don't prevent players from sacrificing creatures.
Removing a shield counter when a permanent would be dealt damage or destroyed isn't the same as regenerating that permanent.
If a permanent that would be dealt damage has more than one shield counter on it, that damage is prevented and only one shield counter is removed.
If a permanent with a shield counter is dealt unpreventable damage, that damage will be dealt and a shield counter will still be removed.
A creature with a shield counter on it may still be destroyed by state-based actions if it has damage marked on it equal to its toughness or has been dealt unpreventable damage by a source with deathtouch.
"Shield" is not an ability that creatures have and shield counters are not keyword counters. If a creature with a shield counter loses its abilities, the shield counter will still protect it as normal.