Well lets compare two FPS games: Call of Duty and Half-Life.
In Call of Duty, the game pretty much consists of moving from fire-fight to fire-fight, and none of the enemies really differ in terms of difficulty presented. Apart from the occasional enemy wielding a rocket launcher, you're never going to experience variety. Now, they can create great, cinematic set pieces, but the only thing that sets your PACE through the levels is the difficulty settings.
Now compare that with Half-Life:
First, in the Half-Life series, you have sections of the game where the combat is broken up by physics puzzles. I think the best example here is the level Sandtraps where you have to create a path along the sand using planks and crates or be set upon by the antlions. Not only does this slow you down, but it also adds dramatic tension; you really DON'T want to touch that sand and it makes you anxious.
Second, the Half-Life series has a number of set pieces in the game that change up your playstyle. The game shifts constantly from urban firefights, to frantic running through zombie infested zones, to moving from cover to cover to avoid snipers, to driving along the coast in a buggy, to parts where you need to bunker down with a rocket launcher to take down striders and gunships. All of these parts play very differently from each other, which keeps combat fresh. All you get in Call of Duty is the same routine of "Take cover, pop out and shoot, take cover to let the jelly fall off." Also, Valve has excellent atmosphere in their levels; it can be really scary when you walk into a room only to hear the rattle of the black headcrab to your left.
Third, the Half-Life game, unlike Call of Duty, forces you to seriously manage your resources. In Call of Duty, as long as you don't die, you can hide behind a corner and generate to full life. Meanwhile, the enemies generally don't work TOO hard to flush you out. Wheras, in Half-Life, you have to carefully manage your health and shields. If you accidentally eat a grenade and wind up with 30 HP, you might be in a LOT of trouble, so it forces players to play smart. Furthermore, unlike CoD, enemies are quite durable, so the possibility of running out of ammo is a real thing. Often the game will give you means to defeat your opponents to save bullets (Crush them with a shipping crate, flip a car on them, chuck something at them with the gravity gun, Let Alyx kill them, use a trap) and these solutions force the player to think about how to solve the problem.
Finally, the Half-Life enemies, unlike CoD, present a number of different enemies with unique mechanics to challenge the player. In CoD, your basic enemy is going to stay at medium range with an automatic weapon. Occasionally you will run into a shotgun. Rarely there are snipers and RPG enemies. Combat is generally the same. In Half-Life, you have a number of different enemies. The Combine Soliders present a number of challenges, especially the elite soldiers with their charge shots and the Shotgun soldiers. Headcrab zombies force you to multi-task between the lumbering but powerful normal zombies, the sucidal Combine Zombies, the speedy fast zombies, and the durable Black Headcrab Zombies. God help you if you get bitten by a black headcrab during a scuffle like that. Antlions are highly mobile and try to swarm you and will endlessly spawn unless you either get off the sand or close up their burrows. Striders and Gunships force you to take cover and nuke them with RPG fire. Hunters can be used to mix up the combat by getting in the players face while they try to take cover from other enemies like Striders and Soldiers.
