These shields last until used up or until the next cleanup step, whichever comes first. If an effect prevents a specific amount of damage, it creates a shield that hangs around until that amount of damage is prevented. All damage-prevention spells and abilities are now targeted. When a triggered ability's condition is met, the ability automatically goes on the stack. Its controller chooses all targets for it, and when it resolves, makes all other choices for it.
Triggered abilities can no longer resolve while another spell or ability is resolving.
Phase abilities have all been changed to triggered abilities that trigger when the specified phase or step begins. If an effect instructs you to do something at the beginning of the turn, you do it at the beginning of upkeep. Upkeep abilities are now triggered abilities. There are now two main phases in every turn. They're separated by combat, which is now its own phase.