Choice. Many games provide the illusion of it; fewer deliver it in any meaningful way. Deus Ex: Human Revolution is one of those few: a first-person shooter/stealth/espionage/role-playing hybrid that allows you to overcome obstacles as you see fit. Let's say you require access to a guarded apartment building. You can shoot your way past the patrolling sentries. But maybe you'd rather sneak past them unnoticed, silently knocking them out as you go; hack an electronic lock on a side entrance; or find a hidden vent and shimmy your way inside.