People saying "Just adding health, making damage weaker doesn't change anything" yes actually it does.
Take your average shooter. What happens when you can no longer mow everyone down anymore. You have to use strategy, you have to pick your targets well. You have to conserve ammo. Resistance on hard was a great deal more challenging. Especially the opening part of the church fight.
People saying "It doesn't make the game harder just longer" then what are you expecting? In an fps on hard mode do you want to suddenly have to solve puzzles to board the gunship? Do you want to have to type in geographical coordinates rather then aim your gun manually? FPS games in generally are actually significantly harder today then before. Remember Wolfenstien on the Snes. You walked forward and shot. The only strategies were shoot and walk backwards while shooting, or take advantage of the stupid AI and walk behind a wall for half a second and suddenly they forget you just shot them in the leg.
Today enemies take cover, enemies will take pot shots at you from behind cover and smoke you out with grenades. Killzone 2 has a type of soldier that charges at you screaming and shooting and if you let them get to close they gut you like a fish. Your first play through you don't know when these guys are going to be coming. On hard mode you have to deal with these guys while avoiding your other enemies shots. the same strategy is not going to work.
As for giving them a 'neat trick' what are you hoping for, a one hit kill. Sudden super fast movement, instant dodging? What? These sorts of things have already been called cheap.
So.. to make it harder you can't just amp up the enemy damage and health(because it just makes the game longer), can't give them devastating moves(because that's just cheap), and you can't include environmental effects(because that's just frustrating, not hard)
So, how do you propose they make the game harder while staying true to a genre?
I find whenever a game is hard people just label it frustrating and cheap. Then when a game does a way with the so called 'frustrating' mechanics people say it's to easy and demand a challenge.
To end this anyone remember Super Mario bros Hard Mode? All they did was replace certain enemies with stronger ones, like goombas with those annoying black shell turtles. The time limit was still there so it couldn't have taken much longer, yet it was still much more difficult. Despite the fact all they essentially did was make enemies tougher, which has been repeated here doesn't make the game harder at all.