Yeah, I don't really like this futuristic setting with invented weapons (e.g. that Ka-50/Mi-28 hybrid? Like, WTF?).

IMHO, I found that the campaigns in BI's games got progressively worse (at least from the immersion point of view) with each game. I really loved OFP, thought ArmA was bland (with the newsreels instead of a personal story), but found Arma2 even worse, while the OA and expansions were just terrible. I think it has partly something to do with who the enemy was. While in OFP (and partly ArmA) you were fighting an ideological enemy (e.g. the Soviet Union), it ended up with mirroring real conflicts and fighting some ragtag insurgent units in some desert I couldn't care less about. What's the challenge with all the advanced weaponry against a bunch of barely armed guerrillas in converted pickups, really?

Central Europe, Cold War, 'nuff said. Great setting out of the box, and scads of DLC/expansion opportunity off a single primary release.

Yeap, I agree and I also hate the futuristic setting and really hate the invented weapons such as the "Hamok" or the futuristic weapons such as rail guns. I won't expand on this even because I'm tired of discussing this.

Regarding the BIS games (OFP/ARMA/ARMA2/OA) campaigns I have the following oppinion:
OPF campaigns (CWC and Resistance) were and are BY FAR the best campaigns and the main reasons were the "lovable" characters where the playing could easily get under those "character skins". Those campaigns storylines were also very well designed, thrilling and specially BELIEVABLE.
In ARMA1 (Armed Assault) the setting was very BELIVABLE but unfortunally there wasn't any kind of "symbiosis" between the player and the playable characters. BTW, who was or were the playable characters and what are their names?
In ARMA2 (first release) the campaign seem and did have lots of interesting features such as "detailed" characters there was a considerable level of work into trying to get the player into the "character's skins". Besides this campaign was Co-Op playable (up to 4 players) which is IMO a major PLUS! But unfortunally this campaign had LOTS OF BUGS in it's initial release, the map of Chernarus was (and still is) very heavy on the hardware and in the end we had the WARFARE missions which completly broke this campaign for me. Most of Arma2 campaign missions that didn't use WARFARE were quite good inded when the bugs weren't present, that is! This campaign was considerably shorter than the OFP campaign.
The Operation Arrowhead campaign was IMO perhaps the best campaign after OFP because you could play diferent missions in diferent roles such as Infantry soldier, SpecOps, tanker and pilot and it was Co-Op playable and finally it has IMO a very good plot/setting. Unfortunally this campaign was VERY SHORT (7 missions max) and the charaters weren't very detailed and "lovable" and there isn't any significant "symbiosis" between the player and the playable characters in this campaign.
The BAF (British Armed Forces) campaign had IMO a good plot and interesting missions and there was a considerable "symbiosis" between the player and the playable character (Brian Frost) but unfortunally this campaign was TOO SHORT (4 missions only!!!) and it wasn't playable in CO-OP!!
The PMC (Private Military Contractor) campaign was IMO a major VOMIT!! So I won't elaborate any further regarding this campaign.

Regarding ArmA3 campaign: Futuristic setting with futuristic/imaginary weapons and a campaign that isn't playble in CO-OP and aparently it's your character alone against an entire enemy armed force which dominates the "world"(island) with some RPG elements of ArmA2 initial campaign. Humm, we'll see but so far for me it's a big: NO thanks....

Preview materials clearly state "Evolve from hunted prey to military commander".

Would be too much fun for an FPS: cold war gets hot==>"instant sunshine" released==> game over

Not exactly.
There were large dozens of conflicts which were "by-products" from the cold war. Examples of these are the Indochina and Vietnam war (which IMO would be great scenario to be modeled in ArmA games), Afghanistan (Soviet intervention), Korean war, Iran Iraq war, Pakistan versus India conflicts, several African conflicts such as the Portuguese Colonial War or the Bush Wars (South Africa versus Angola), etc, etc, etc...

And there were also many covert operations that happened during the cold war which many of them were far from being "cold".
Also during the "cold war" period there were also other conflicts which weren't related to the cold war (at least directly) such as the Falklands war.

So as you can see, the "cold war" has PLENTY of settings/scenarios which could inspire an "ArmA3 campaign" (or any other future ArmA game campaign) that doesn't envolve "instant mushrooms of death"!

Besides the OFP:CWC and Resistance campaigns modeled NATO versus Warsaw Pact campaigns that while imaginary they were at the same time VERY BELIVABLE and again without the need of using the "instant mushrooms of death" (well at least or unless you lost the CWC campaign that is).

As long as the game engine includes real weapons, I am interested. If ArmA 3 doesn't come with M-16s, AK-47s and the conventional USA/Russian troops that use them, it offers nothing for me no matter how much the engine is improved. Sure modders can backfit the stuff I like and want, but when I buy a game, I prefer that it actually be something I like and want to play without relying on mods to give me what I like and want. I never played any of the campaigns. I liked single missions and preferred multiplayer with close friends on a home LAN. ArmA 2 is pretty cool, but the delivered missions (single and multiplayer) have never equaled the enjoyment I got from OFP/Resistance. The user made missions and addons for OFP have never really been equaled either. I still have 6 PCs that can run OFP for hosting LAN parties with my old friends. We always play certain stock missions and a few user made missions that we have never grown tired of playing and replaying. I have tried the mods that allow retrofitting OFP content into ArmA 2. It is cool, but not as cool as playing the original game. Give me all of the CWC/Resistance material upgraded to ArmA 2 or ArmA 3 standards, and I would be one happy camper. I am still waiting to see what ArmA 3 ultimately supports, but from what I see right now, this will be the first sim in the series I don't buy.

As long as the game supports realistic ballistics and weapon modelling, and overall a better virtual combat experience, I'm sold. 5.56mm/7.62mm bullet rounds will still fly the same in 10, 20, 50 years from now.

I gotta ask, Why do people think a Search and Destroy (Vietnam) Military
conflict would make a good FPS?

For me, personally, because it has the potential for enthralling and immersive missions. Operate in a hostile environment (in other words, keep your head down for as long as possible), seek out the target, destroy it and vanish again.

Far more interesting to me than taking half the freakin’ world on or fortifying and fighting back wave after wave of onrushing enemies.