In terms of the "older games being harder", sometimes I think that's just nostalgia coming out from when we were kids. We weren't as good at games back then. Doom may have seemed difficult then, but I think if you played it again you may be a bit disappointed. That isn't to take away from Doom, because it was an absolute marvel and achievement for the FPS genre. I grew up playing that game. I'm just not convinced that it's difficulty is as extreme as you're making it out to be.
Yeah, I've gotten this arguement from people before, but it falls flat for me for the very simple reason that I frequently still actually play all of those older titles. It's not just nostalgia in my case. Hell, you should see my setup here.... I've got the two screens and the PC and all these damn cables..... and then right in the middle of the desk is this ancient Atari 7800 unit. It's not there just to look pretty, it's there because it gets alot of frequent use (though mostly with the 2600 library, of which I have a gazillion). Cartridges everywhere. The same goes for the NES and a few others. Heck, I bring some of the NES games with me when travelling at times, as I have one of them funky gizmos meant for playing them on the go.
So their frequent use is where I get all of that from, not just mere nostalgia. Quite a number of the games I have now I actually didn't have way back when, as I couldnt buy them on my own back when I was a kid, but now I can grab them when I want them, so I keep getting more.
I'm not one that would count on pure nostalgia for anything anyway; my memory is too messed up for that.
Granted, the games back then arent like, really stupidly absurdly hard, but part of my point is that alot of players, taken from THIS era and given many of those older ones to play, often think that they ARE really stupidly absurdly hard. WHich showcases the low difficulty of many games these days. It's also why I myself tend not to be very interested in alot of mainstream titles. As far as I'm concerned, easy=boring.
It's true that most of these modern FPS games attempt to be realistic but fail in some (or many) fundamental ways. It's also true that their "single-player campaigns" are extremely silly and pointless. However, to be fair, most people know that these games (CoD, Battlefield, Halo, etc.) are made more for their online multiplayer aspect than anything else. The single player campaign is really just a tertiary concern. You could argue that the multiplayer aspect is really subpar as well, but that's where most the production values went, so if you want to judge the quality of the game, you should probably focus on that.
I'm curious about what you think of a game like CS:GO? There is no single player campaign to speak of. They didn't waste their time or pretend to make a decent campaign (which we all know Valve can make). They simply stuck with the multiplayer aspect and let the game stand on its own merits.
Taking a look at all your requirements:
1. There is no pointless "cover system". You can't lean out from walls or press a button to "stick" to a box. You can hide behind walls to avoid fire if you want, but you can do that in any FPS game (including Doom). However, many of the high powered weapons can penetrate through think surfaces, so often times even that won't save you.
2. You can't regenerate health at any time during the round. Once you've taken damage, you stay permanently hurt until the round is over or you die.
3. You don't respond until the round is over, so there's a realistic portrayal of death and loss. If you die without inflicting any damage on the enemy team, you've just become a a serious burden on your own by basically giving them an extra enemy to fight.
4. All your opponents are human, so you can't complain the the "AI is dumb". The matchmaking system pairs you with people (on your team and theirs) which are similar to your skill level. Typically you'll be pushed to your limits every game.
5. There are ~20 official levels, with more coming, and countless user-created levels to play on community servers. Granted, the levels aren't huge, and you'll end up playing on them a lot, but even after a dozen times, there are still new places to find, tricks to discover, and strategies to learn. There's much more to discover, for example, than playing the first level of Doom 2 for the thousandth time.
Sniping is there, of course. Aren't you the one who wanted realism? Though it actually requires quite a bit of skill to die against good player because the zoomed out crosshair has been removed. In addition, sniper rifles are extremely expensive to buy, and don't offer the same monetary rewards for killing people as the other weapons. Cheap items like Flashbangs and Smoke Grenades make it much easier to deal with these players when necessary. If you kill them as well, they are at a huge disadvantage because they'll be broke.
Yes, you can only carry a few weapons at a time (per round), but every new round allows you to purchase a whole new set of weapons. With a whole assortment of pistols, shotguns, sub-machineguns, machine guns, assault rifles, sniper rifles, and grenades to choose from, your arsenal is much larger than in any single-player FPS I've ever played.
So I think there are games that strive for realism and do it successfully while still keeping things balanced and fun. Not all games are the CoD/BF/Halo equivalent, and I agree with you, those games are awful. I just disagree that it's the only thing we're stuck with.
Oh, I'm not saying that literally ALL FPS games these days are like the ones I was ranting about.... but it's enough of them to be by far the majority, and even those that dont exactly follow those annoying trends are at least influenced by them, which is still..... obnoxious.
For single-player VS multiplayer, I dont generally compare the two that much. I'm mostly thinking of the actual mechanics instead, unless considering the actual design of the levels themselves (the linearity and such). I can only say so much about the multiplayer, as this was always a single-player genre to me. Multiplayer in one of these, for me, would be..... ehhhhhh. Let's just say I'd likely not be very good at it. My skill in the Doom-style ones (and in so many other types of games) is mostly the ability to dodge absolutely everything, even at point-blank range when it doesnt necesarily make sense to BE at point-blank range (punch the Cyberdemon!!!), and the point-blank part comes as a result of my horrible aim otherwise. Think of Rambo, spraying bullets everywhere at random angles, except many times more inaccurate. Many, many times more. I always say, I couldnt hit the broad side of a barn (or any side) from inside the barn. Cant aim worth a crap. Though me being the way I am I just charge at everything anyway, so that works out well enough.
But yeah, I can only say so much about the multiplayer experience. CS in particular I'm not at all familiar with, so I've no opinion overall on that one.
As for the bit about realism..... nnnnnno, I'm pretty sure it wasnt me that wanted realism here.... I was just complaining about how CoD and it's brethren CLAIM realism but then put in elements like soldiers standing still for 10 seconds behind a box, allowing them to totally heal multiple gunshot wounds.... stuff like that. But I myself dont actually care for or need realism in gaming, really. Doom and such, for example. "Let's fight the endless armies of Hell with a shotgun!!!" always sounds way more interesting than "do soldiery things", which usually sums up my thoughts on military-type games.