The HUD and the first person arms and weapon are hidden.

Headshotting an Unggoy results in an explosion.

Rare combat dialogue becomes more common.

Enemies throw grenades more often

Motion tracker is disabled.

Enemies have increased health.

Acceleration from explosions is increased.

Major upgrade to the capabilities of enemies.

Enemy resistances and weaknesses are increased. This also affects the player.

Enemies always go berserk, always dive out of the way, and never flee.

Energy shields don't recharge unless the player melees enemies.

Weapons drop much less ammo.

On campaign solo, player death results in the mission restarting. On cooperative play, player death results in reversion to previous checkpoint.