It seems to be universally agreed the 'real meat' of a FPS game is its multiplayer component, and usually (and unfortunately) the SP campaign is lackluster, average.But there are a few FPS games that a lot of people agree have an awesome SP campaign, like Half Life, the original Halo, etc..

Anyway, in your opinion, what´s necessary for a FPS game to have a great single player experience?...why is it the vast majority of FPS games seem to have such a poor/average/decent at best campaign?

A decent plot, great gameplay (not just a bot-practice mode for multiplayer) and meaningful content and/or a bit of variety.

Look at Metro: Last Light. A FPS game without multiplayer component.

Copy what Bioshock did

Variety.
Good story in an interesting setting.
Freedom to explore the surroundings and game mechanics.
Pacing, good balance between quiet sections and action.
Memorable characters along the way.

Everything that HL2 did basically.

Because too many of them try to be shooters rather than tell a story in the first person. Even Bioshock Infinite that has a great story to tell is bogged down (and in my opinion made sub-par) because of its shooting mechanics that take precedence over everything else. It takes away from the story, it kills suspension of disbelief, ruins pacing and is just boring. The best FPS campaigns have as much interaction from the player as possible: meaning cool setpieces, variety and few (preferably no) cutscenes that break the pov. Really this goes for all games, but the action of shooting a gun becomes very mundane in FPS's. Rather than making killing a human being intimate, they throw hundreds of faceless enemies at you - it makes for even less interesting gameplay when the gameplay is already uninteresting.

nu-uh, copy what resistance did!

Pacing, don't have nukes and explosions going off every 5 seconds, that ruins the impact. COD4 built up to the nuke which made it incredible to watch.

I'll use Far Cry 3 as a primary example since I'm playing it right now.

a) Make Quicktime events go and die. When a scripted even occurs right after a cut-scene and the words "Mash C" appear, do the developers really, honestly think it's fun? Is it supposed to be immersive, this mashing of a key? Is that what my character would do if really attacked by a shark, mash the hell out of random key? Which, usually, is used for the 'crouch' action. Am I crouching at the fucking shark? Crysis 3 was also really annoying for this. "Oh, long cut-scene, I'm gonna put the controller down and watch ... what the hell? I have to suddenly spam X? No, wait, wait ... argh."

2) Don't force scripted encounters where the player HAS to do X, Y, and then Z for arbitrary reasons. So I'm playing Far Cry 3 and the island queen or whatever the hell she is has me take a drug. Next thing I know, I'm having to fight a giant monster with a bow and arrow. I actually hate the bow and arrow and never use it. I want my bloody guns back. It's like some kind of boss battle in a different genre, I'm having to run around this small area, dodging magic ink balls, while attempting to shoot it in the head with arrows. Absolutely stupid sequence.

c) Random vehicles that I will only use once, just for this level, and which I have to learn about on the fly. No, thank you. Killzone 3 was bad for this. Most of my deaths came not when I was actually SHOOTING STUFF but when I was doing these vehicle levels.

8) Let me retain control of my character while I'm being talk at by an NPC. Far Cry 3 does this stupid ass thing where I can control the character's vision a TINY bit with the mouse but it's all floaty, like he's high on painkillers. The hell is that about?

I like freedom and versatility. I like side mission that feel important yet aren't like "kill random pirate captain X with a knife while he's peeing". I don't mind being on a ride but I want to be able to move around a bit while I'm on that ride. The game should be controlling the EVENTS, not ME. Let me control me.

Campaign is pretty much the only thing I buy FPS games for; online has no appeal to me.

What I've noticed about the ones I enjoyed most this gen:

- Good mixture of stealth and all-out action.
- Smooth pacing; I dont want to spend five minutes walking through a playable cutscene.
- Variety; vehicle sections and such go a long way towards spicing things up.
- Feels empowering; Goldeneye Wii was masterful at this, really making you feel like a badass spy outsmarting your foes.

there's been a few good single player campaigns like bio shock halo and lesser games like resistance

it depends really, a cod campaign would be out of place if it was like bio shock or metro

actually I'm liking killzone shadow fall campaign so far, it's not world beating but it's well worth a play through