The setting, enemy, and weapon set is always the same. It'll be level 3 before you get a sniper rifle, level 6 before you get a 'nade launcher, level 10 before the mutants show up (or maybe not).
I like FPS games that take the genre and move it somewhere new, like Farcry2 is a very different sort of FPS, kinda like FPS meets GTA. Call of Juarez as well, sometimes it's nice to play a game where there is no chance of zombie infestation.
Op. Flashpoint 2 has you fighting the Chinese, and seems to have quite a well thought out plot - but really what is a plot, really. What if they just made a red army and a blue army, and the aim is just to wipe the other army out and take out their hardware then get to an exit. It might seem like a dull plot for a game, but that's pretty much any mission on any FPS game ever made. Plot is really just there to make it 'interesting', reading more into it than that is pointless, at the end of the day the only people who get a really rough time of it are the zombies.
Personally, I'm still waiting on a decent Vietnam FPS, CoD:Vietnam for the love of all that is holy! - That'll be the next CoD game I buy, hows THAT for a boycott!

I don't need to shoot any more Germans, I don't need to run through sandy sand levels made of sand armed with yet another M4 - RPG elements can make a big difference, but as a whole, FPS games need to mature a bit, it's actually less fun when things just go over the top. When a bodypart comes off, it should be because someone shot it in just the right spot - it should be a rare event - and maybe most controversially, it shouldn't always kill them. When you see a war movie and somebodies had a bad time, and lost half their leg - they aren't dead, they're in bloody agony. That'd be pretty disturbing, to 'nade spam a bunker, then go inside to find people screaming in agony. You can plough through pedestrians in GTA, yet not one of them ever seems to suffer that much. It's more comical than anything - Op. Flashpoint2 is quite realistic, shame they didn't think to include disturbing scenes of human suffering, amputation and torture.
Bringing up FC2 again, because in multiplayer it actually has quite a cool feature - when you get put down, someone can revive you - so you put people down, and leave them out there bleeding to death - Johnny Hero comes along to save the day, and ends up chewing lead as well. It's pretty good fun, you can even put people down then torch the grass so they roast to death. In Call of Juarez (first one) you can go around with a bible in one hand, reciting passages like Samuel.L.Jackson - while you have a revolver in the other hand - I'm not sure that's what they had in mind when they came up with dual wielding.