S.T.A.L.K.E.R.

The gunplay is actually good now. Enemies don't take too many rounds to kill, and your guns aren't just spitting lead anywhere but where you're aiming. Mutants are still tanky, but they are meant to be. A dude wearing a sweater goes down quick. Guys in exoskeletons also don't need 3 magazines of ammo to take down anymore.

And it's entirely open, with the biggest map in the series so far. No more loading between levels and being forced to transition at specific points.

Weapon modding is a bit more fleshed out than the previous 3 games. There's the usual tech upgrades, but then there are more attachments for more weapons, and even unique attachments for many (maybe even all) the weapons. There are also unique weapon variants that are more than just a named gun that looks and operates the same as all the others. They may have a special skin, or even unique modifications.

Emissions actually alter the zone in a real way now. There are also new kinds of anomalies. They will shift around after emissions. I had run into a building to avoid one, and upon exiting, a Glass Shard field was right outside the damn door.

The mutant AI is different, too. I only ever really worried about bloodsuckers and controllers in the originals; but now everything actually will fuck you up and seems somewhat intelligent. The animals circle you and stay on the move while you're aiming at them, really only coming at you from behind, dwarves are hella nasty now with the ability to take you gun and use it against you, and even bloodsuckers are more vicious always trying to get you from the sides or behind. Some of them also have a poison attack.

Overall it feels like what the first game was really wanting to be, but was limited by what they could do back in 2006.