First, it won't be about "how bad is CoD or Killzone and how much other games deserve to be called 'the best gaem in teh univerce'", it will be rather about what I think people want from FPS.
Short note, if you disagree with something or you want to expand it, just leave a comment, please.
1. Jump - it may sound like a joke but it is simple as that, player is needed to be able to jump, even if there is no place to jump onto. People just think that it gives much more freedom. On the other hand it turns some MP experience into rabbit hunting but few games managed to help it after previous instances were flooded with bunny hopping.
2. Basic gameplay mechanics need to be at least decent and balanced. It doesn't have to be anything fancy, but shooting at enemies has to be fun, at least to some degree. And adding too much new stuff may turn against designer as people will find everything repetitive after a while but if they have some new unique features, they will focus on them and say that they are repetitive. But mechanics can't be bad enough to people throw away from the game.
3. Story - whatever story you will create, you will find people that will find it dull, boring and "I've already seen it". So at least here, just create whatever you find fun. You could create something about space monkey farmer going insane, invading some research station where they were working on space-time continuum weapons and wrecks the space-time continuum itself to create weird spaces and bring some unexpected enemies together and at the climax you have enormous battle between vikings with lasers riding dinosaurs and Napoleon's army enhanced with new weapons and cyber implants. Or you could have story about a man waking up in middle of some riot, trying to find his family, only to discover that there is no wife, no kids, and is just a part of some mega conspiracy to throw government to its knees. Whatever you'll do, some people will find it either stupid, too serious, boring, being another variant of some other story.
4. Length of SP - currently some people are happy to have game 4 to 8 hours long and that's it. They really don't need longer game. And people will buy short games if they have MP. But if you don't want to add MP, prepare to create looooong and non-linear game. Otherwise it will be lost.
5. Ah, yes, linearity - short game is fine with linearity, longer one will loose all replayability if you will have to do it the same way.
6. MP - if you're not going to have long SP mode, add at least team deathmatch and some co-op mode and maybe some variation of objective-based one (even capture the flag will be enough). Otherwise people will complain. And don't forget about levelling up, unlocking new stuff, new stuff that is important and could change the way players play.
7. Graphics, sound, animations - make it at least decent. People love great graphics but will always find something that doesn't work. And the more you will try to make some things unique, the more likely they will point out all things that won't work. And that goes mostly for animation systems. People really don't find anything with the way characters move currently (which is sliding, having no transitions between different gaits, etc). Although if just running animation looks nice and there are lots of level-specific animations that are played in just one place, people will love it so focus on that. But on the other hand, great graphics could sell the game (if the gameplay is not completely broken).
8. AI - it should work, just work. If you want to have heavy scripted short SP (and go for MP), then really don't care too much about AI, but with good approach, you could have less scripts and less linear game. And again - people will always complain and always will find something that didn't work. Oh, use lots of sounds. If people hear that AI says something, they already assume that it is intelligent. But of course give at least shadow of what was said - ie. if AI shouts that they are going to flank enemy, they should move, even to predefined points or maybe even randomly. They will flank player eventually.
9. Replayability - after finishing SP, you should have all weapons unlocked and maybe some extra bonuses added. But now there is a twist, if you finish the game for second time, give some more stuff to player. I finished Dead Space twice, although I didn't actually like the game too much, but after finishing it for first time, I had my stuff powered up and I had new rig unlocked and... was really disappointed after finishing it for second time when I received nothing new. Even if it breaks gameplay and makes player god-like, it will be still fun for players to play the game again. Even survival horrors. And that's it, it is in many cases not about doing things in different way, as many people will just do the same thing over and over again, but giving new stuff (this will actually force people to try new things). Of course some sandboxing, even on small scale, will help.
10. Innovation - have something unique and innovative in your game. It doesn't have to be something that will redefine the genre. Probably there isn't even such thing as, again, people will find it just a twist, idea that appeared somewhere else, or they will find it unnecessary or even dull or stupid. It is better than to leave such thing. But still, having something unique may help your game to stand out.
11. Create sequel - if first part was at least part-successful, at least critics liked it, go for a sequel. And if game didn't go as well as planned, think not only about game but also about marketing (these guys should do it themselves but you also speak, don't you?), maybe you told too much? Or hit wrong tone?
That's it. It's that simple, although having good story, good MP, good mechanics etc. is difficult, but you really don't need to invent wheel all over again.
And just one thing - games need to be fun and can't throw away players. The more people will find game to be okay when they hear about it, the more will try it. The more will have fun playing it, the more will tell their friends. The longer the experience and more things can be done (either by having complex, long and fun SP or balanced and fun MP), the less likely they will sell it to someone else.
