ArmA apologist here.
ArmA 2 CO is currently an incredibly solid game, once you learn the subtler bits of it. Some things that seem game-y when you experience them are more realistic and authentic than you might think.
Everyone hates on ArmA's UI, but no commo-rose type system can approach the fine control offered by the tiered menu system. Contact, 2,3 go prone and deliver suppressing fire on that target in the bushes, 4 crouch and watch northwest, 5,6,7,8 go combat movement and bound to that rock then engage from cover. I challenge you to find any FPS with such fine control for a leader. Voice control using something like Voice Activated Commands (VAC) makes it even sweeter. I don't want to be limited to ALL, ATTACK MY TARGET. BOAT SPOTTED. BOAT SPOTTED.
I find the addition of in-mission gear switching kind of sketchy, since if you have a 10x scoped rifle, you probably shouldn't be kicking down doors in the same ten minutes, and you'd probably cry yourself to sleep before you removed that scope and possibly wrecked your zero. Of course the limits of the game environment often make that needed, so it's a good thing all around, especially for mod makers.
The addition of a real vehicle/world physics engine and a real inverse-kinematics (ragdoll) system are the largest things IMO. This will make so much more possible -- real rope insertions, real cargo lift helicopters, proper vehicle crash and destruction simulations. IK will also make modding a million times more enjoyable, as the current 'weighting' system is frustratingly out of date and awkward to work with, especially when animating new models.
Anti-material rifles (and all weapons actually) already feature penetration and deflection, which is controlled by the material of the object, the caliber of the bullet, and the velocity. Shoot tracers from the M2 into a concrete wall and watch for the deflected shots. Alternately, line up several concrete walls in the editor and shoot through them, then look for the bullet holes through each. I was just testing this for a silly extreme penetration sniper rifle mod I was working on, shooting guys behind an IFV through the hull of the vehicle. Some sort of destruction ala BF3 but less ridiculous would be nice, but that's not in the pipeline I don't think. The engine already includes anti-material components, but sometimes it doesn't seem to be right. For example, you can definitely disable the turrets on IFVs with the .50, but for some reason I can never shoot out the engine block of the UAZ. And from modding, I know that the system for that exists inside the game already, and is implemented spottily for some vehicles. Not sure if it's just buggy or what.
ArmA already has a pretty impressive stealth simulation in my opinion. It doesn't play like Splinter Cell, because that doesn't make sense and isn't realistic anyway -- you can't just stand behind a dude for two minutes and not expect him to 'feel' you. Try it in real life. With a melee mod, I have successfully crept up behind two guards in ArmA and taken them out. The first began to turn just as I got to him, but the rifle butt to the face handled that. The crack alerted the second, but I was able to draw my pistol and kill him just as he got his rifle up and a burst off (a miss). The existing stealth is solid if you play in a cautious, tedious, realistic way. It is punishingly non-game-y.
ArmA's mission editor is brilliant and one of the most accessible ones I've ever used. Place groups of guys, a vehicle here or there, support trucks and a medical helicopter, give them waypoints, set players and playable, and boom, you have an instant mission to play by yourself on any terrain. The scripting language is forgiving and easy to learn. ArmA 3 will use Java, so that'll be even more powerful (and more familiar to some). A sophisticated sim like this is never going to be fully point-and-click for complex editing IMO.
Since the 1.6 patch the ArmA AI are highly improved. They will rarely step out into the open except when they are not aware (i.e. the mission maker has poorly configured them). Their combat movement, bounding, peeking around corners, use of suppression/grenades, etc. is all quite strong, and their rifle accuracy is pretty human -- if you can shoot them as a player, they can shoot you just as fast at that range. It's even better with some of the small tweaks done by scripts/mods like UPSMON, ACEm and ASR AI. Ask the SimHQ team, who gets flanked and mowed down by UPSMON AI week after week! I expect the ArmA 3 AI to be even more polished with the new refined movement animations. Certainly no game has comparable infantry AI on the sheer variety of terrain (from open fields to MOUT) that ArmA's AI can handle.
I am also looking forward to Render-to-Texture (confirmed in Take On Helicopters) and also underground areas (I'm hoping, since it's in VBS, and some ArmA 3 shots show hill-side entries to bases). If along with render-to-texture (MFDs), clickable cockpits are included in ArmA 3 like in Take On Helis, then you can expect some serious mid-fidelity aircraft simulations to come out of the modding community.
Bugs and all, I expect this to be brilliant.
"It is necessary to develop a strategy that utilizes all the physical conditions and elements that are directly at hand. The best strategy relies upon an unlimited set of responses."
-- Ueshiba Morihei