But your assessment of FPS games isn't strictly true. What was the last FPS game you played? It's like saying, all you do in Monkey Island is point a cursor at objects and click on them.
If you're thinking of Left4Dead (you mentioned zombies), its game mechanics are: run to the safe house alive - zombies will run at you, therefore shoot. That's probably closest to the kind of gameplay you describe, but it's main purpose it to be a zombie survival game, so you've got guns, you've got hordes of zombies and you're supposed to survive them.
Borderlands? Yes, you shoot and kill, you also earn money, you can buy better weapons, you've got a nice selection of weapon choices, you've got vehicles you can drive in to get from A to B, sometimes you have to fight other vehicles, missions may require you to kill something specific or to acquire an items. Certain bosses may require different methods of killing them, you've got a levelling up system, so you can customise your character and pick out their biggest strengths.
You can approach an area in a number of different ways, you can go in like a brute and guns blazing and shoot everything. You can be more stealthy and finding a vantage point and snipe everybody out, you could go in stay behind cover and take out foes one by one. Or you can just mix it up. There's varieties of enemy types and they're not all coming at you at once, you meet new ones as you progress. There's also a plot line to follow and your missions are centered around that plotline. Different areas can pose different advantages and weaknesses, so if you prefer 'guns blazing', you might find your enemies have lots of cover, so you adjust.
The guns you choose, they can possess different kinds of attributes, it's not as just clear cut as "shotgun", "pistol", "machinegun", because you might have 2 shotguns, but they offer completely different functionality and may exploit different weaknesses in an enemy. So when you're collecting your weapons you'll choose a load out that best fits your strategies and best fits the kind of foes you're up against.
The level designs offer a bit more variety so you don't get bored of the game play as you go through and you're faced with different kinds of challenges.
So it's not really, "point, shoot, throw grenade". There's a lot to Borderlands that differentiates it from a game like Left4Dead.
Solving puzzles in Monkey Island is not that different from solving puzzles in other P&C games. It's always "use this item with that item" or "give this item to this person" and whilst they are great games to play, but if the criteria you're using to judge FPS games to be the same was applied to P&C games, then they too are all the same. But I know they're not (and I love the genre). Broken Sword is not the same as Beneath a Steel Sky.
