Well, the way I see it is fairplay to them for doing something differently!
Sure, it's nice to have it down to earth and realistic, but ArmA and OFp were based on fictional (inspired by true events, sure) events. They are just going that bit farther this time. When you have ArmA 1 and 2 baased around roughly the same time, I'd begin to wonder what difference in 3 would make me buy it. I mean, the game is going to run worse on my rig, if anything, so why not just stay with ArmA 2? Thankfully, they are trying this future approach. It's not as over the top as many other games, it's feet are still fairly planted to the ground. Thankfully, the mod community will make things the way the fans want as they always have, so either way, we will get both time periods.
From the screenshots and what's been said I'm hopeful. But these kind of previews have been at the start of ArmA 2 before and that turned out not to be a massive overhaul over ArmA 1 as it was essentially the same engine, it played very similarly. I am particularly worried when I saw supposed system specs.