Gameplay
The different "pillars of gameplay", as called by the developers, are "Combat", "Stealth", "Hacking", and "Social". Players can switch between these gameplay types whenever they please, and certain pillars may flow into others. For example, a failed hack may sound an alarm and start a fight, and social skills might lead to the player gaining access to certain areas more easily than otherwise possible (thus avoiding the need for stealth or combat).
In the series, Augmentations are technological modifications to the human body that grant the user superhuman abilities. While augmentations in the first two games were based on nanotechnology, Human Revolution instead features mechanical augmentations as it precedes the first games chronologically. As players progress through the game, they can activate augmentations catering to each of the four gameplay types. These augmentations are unlocked by either earning enough experience to level up or by purchasing an item, known as a Praxis Kit, from L.I.M.B. clinics earning the player a distributable skill point called Praxis.
While the player character is highly capable of bringing death on his enemies, the player is never forced into acts of lethal violence, except during boss fights. Therefore, the use of lethal force becomes an ethical choice for the player. In fact, players are awarded the Pacifist achievement/trophy only by completing the whole game (including the prologue chapter) without any NPCs dying by the player's hands. This includes not only direct actions such as the player character shooting and killing an opponent, for example, but also indirect methods like re-programming security robots or turrets to fire at and eventually kill the player's enemies. Even accidental kills like the player character performing a non-lethal takedown resulting in the NPC falling into electrified water or off a ledge make it impossible to earn Pacifist unless the player reloads a previous saved game. The only opponents the player may kill and yet still receive Pacifist are aforementioned bosses as well as non-human NPCs like security robots.
Augmentations, while enhancing the player's performance in each of the gameplay types, also allow players to craft their own methods of play as they see fit. For example, players can prioritize augmentations that either improve the player character's combat prowess or his hacking abilities while (mostly or completely) neglecting the other without ever being unable to complete a mission objective.
Weapons fire distinct ammunition types instead of depleting a unified pool (as was the case in Invisible War). They can also be upgraded like in Deus Ex via a variety of so-called "weapon mods" to improve their performance, such as reducing the time it takes to reload, increasing magazine size, adding a laser targeting device for increased accuracy, and so on.
Deviating from previous titles in the series, Human Revolution uses a regenerating health model. This change was made because the developers did not want players to get into a situation where they were unable to progress due to low health, and would be forced to "scrounge for med packs" and food. This 'scrounging' breaks the flow of the game when the player retreats to search the entire level for medical supplies. In combat, only brief exposure to enemy attacks is necessary to kill the player, so regenerative health is only a major factor between fights, not during them. The player can, however, use medical supplies (such as painkillers) and various alcoholic beverages to regenerate the character's health and even boost it up to twice the normal amount.
Similar to regenerative health, the game also features a new regenerative energy system, deviating from the previous title's use of items to restore energy. While players will still need to use items to boost their energy cell charge past one, the first cell or any cell partially full will gradually recharge. Despite upgrades to the energy recharge portion in the skills, ultimately only the first cell will recharge by itself when completely depleted.
Another major change seen in Deus Ex: Human Revolution is the highlighting of objects a player can interact with, explained via the in-game plot as tactical vision Augmentation. This feature can be seen in the gameplay preview released in March 2011. This highlighting of objects can be turned off in the game options. Human Revolution is primarily a first-person game, but switches to a contextual third-person viewpoint when using the cover system, climbing ladders, activating certain augmentations, or for melee combat.
Read more about this topic: Deus Ex: Human Revolution