The controller's light bar will "reflect the light gem in the in-game UI" during gameplay to reinforce whether the game's protagonist, Garrett, is safely stowed away in the shadows. The bar will brighten as a player leaves cover and darken once they return to the stealth-favoring darkness.
The DualShock 4's motion tracking can be used with Garrett's bow, allowing players to physically aim arrows at their unlucky targets. The controller's central touch pad will also be used to examine the map or pick a new weapon from your arsenal.
This is Thief, of course, so its fondness for player choice involving combat means you don't have to kill everyone. If you go that route, however, you'll be able to pick your weapon of choice without bringing up a menu that covers the main screen.