1. Core Mechanic
Roll 1d20, apply modifiers, compare to a target number. Result depends on whether action is opposed or unopposed.
Opposed (any situation where the reactor is capable of action): Actor and reactor roll 1d20 and apply modifiers to the result. If actor's roll is equal to or less than reactor's, reactor wins. If actor's roll is better than reactor's, actor wins.
Unopposed (any situation where the reactor is unable to act, or is an inanimate object): Actor rolls 1d20 and applies modifiers to the result. If actor's roll is less than the reactor's, actor fails. If actor's roll is equal to or better than the reactor's, actor succeeds.
That is, when the reactor is capable of acting on his own behalf, ties go to the reactor. When the reactor is not able to act on his own behalf, ties go to the actor.
Glossary
Actor: The one who takes action. The active party, the one with the initiative.
Reactor: The one who reacts to what the actor is doing. The responsive party, the one who lacks the initiative.
Thoughts?