I've played a lot of first person shooters, and while they take place in varied environments (space, WW2, Lairs of Evil Genuises) they play similarly. You go, you shoot, you kill...rinse the blood off your suit or armor and repeat. I'm going to spend the next few blogs on the differences in the game types of this genre.
And to kick this off, I'm going to talk about gun limits. The basics are this: Some games let you have every weapon in the game and somehow lug them around (Wolfenstein, Goldeneye 64, Perfect Dark), others let you have a few weapons (Bond Nightfire let you have 4 plus grenades and mines), and finally some just let you have two (namely Call of Duty and America's Army). There doesn't seem to be a whole lot of story-based logic to this. Wolfenstein and Call of Duty are both WW2 shooters, yet in Wolfenstein the player can lug around a gattling gun, RPGs, various submachine guns, pistols, and a dagger (Special Service guys are really fit), and in Call of Duty, the heaviest load you can get is a bazooka and a Tommy gun with a barrel magazine (which oddly enough the Russians get, but the Americans are stuck with clips?). And between Goldeneye 64 and Nightfire, Bond stops going to the gym, and can only lug around four guns.
Now, there is rhyme and reason to all this (I hope at least, I'm just throwing out theories here). In older games, especially console games, controls were more difficult and aiming was harder (on the N64, aiming anywhere but dead ahead required you to stop and hold down the R button while moving the analog stick). Also, games like Wolfenstein were difficult, and the extra weapons you got had limited ammo and use (like the Panzerfaust for killing SuperSoldats). More modern games focus on aiming and tactical movement, as this is what makes the multiplayer so much more fun than single-player. Instead of running at you shooting, enemies hide behind boxes and throw grenades (More grenades have been thrown at me in COD 2 than were used in WW2). If you had 4 or 6 guns, you could just run in and instead of reloading, switch weapons. Now this is possible with 2 SMG in COD 2, but only effective in Easy mode. Otherwise, you'll need the balance a rifle brings to long range fights.
Less guns means more choices. More choices means the game feels more real and the difficulty increases. It also means you can play fun variations like a "Rifles only" game of COD. FPS should always be about the challenge, because unless you are in the army or police force, chances are you won't be in a situation where you are required to kill in the same way you are in a FPS. There is a twisted, masochistic fun to running low or out of ammo, and the adrenaline surge in knowing that your virtual life is ending if you don't do something soon.