They got it right
I think they got all the elements in the series just right with this game. I feel this game got it just right in terms of a mix of linear and free roam aspects. The game is now played in 3 zones rather than having a more linear path; geographically based where the former NPP was and to the west of pripyat, which were not really covered in the previous games. These zones are large and unlike the previous ones, your 'base' is in the centre of the zone, not in a corner, so there is no clear linear path for you to take. You start by exploring around in one direction and take it from there. You can spend in upwards of 8-10 hours in each zone before all the quests are complete and areas are fully explored.
The side quests are no longer randomly 'kill x number of x' generated ones. They are scripted and designed before hand, and actually provide some entertainment value. The scripted side quests are a welcome change IMO.
There are no longer random anomalies scattered all over the map, meaning now you can actually use your sprint ability without constant fear of getting killed. The anomalies are condensed in specific concentrated areas, which in turn have artifacts spawning in them. They can be good or bad artifacts... but you really have to risk life and limb getting them, or have a kickass suit. While you may say that this is lame, I find it adds direction... explained in the next paragraph.
Recovered weapons are no longer your primary source of income. You can't sell items below ~75% damage. At the start you may find this a real pain and seek some kind of mod to reverse this, after playing through I feel it benefits to the desired role of artifacts in the story. You actually seek artifacts out to sell them, some can sell for upwards of 20k RU, vs about 3k for even the top quality rifle. It means you are no longer hoarding dozens of artifacts in a footlocker in some basement. You keep what you need and sell what you don't... or that is worth selling.
They also refined various elements that add value. The weapon upgrading part is much better done now, as opposed to flash drives you now have 3 kits that you must find and give to the armory guy to unlock full upgrades... basic, intermediate, and advanced kits. These are found with exploration (in preset places). The upgrades are worth doing, and can make weapons very good for the right price. For example the G36 can have 100% accuracy with full upgrades.
There is also an 'achievement' system they added. This is not some gay xbox live thing but rather built into the gameplay. If you do certain things you can get rewarded that permanently changes your game in slight but noticeable ways. For example you can get 'diplomat' which makes everyone friendly or neutral to you except the core baddies and monsters.
And I suppose I should talk about the big-picture things not just the details. The environment is great, and is like the original Stalker. You have a sense of fear and unknown in places when exploring. You come across places that are creepy, and sometimes you come across things where don't really know WTF is going on but continue deeper. Really its the environment you expect in the game. The game world was created in meticulous detail. There are no copy-paste buildings in this game. Every building and part of the map is unique. Even in areas with the same repetitive architecture (like apartment buildings), they still manage to differ small things. There are a wide variety of weapons and monsters that keep you entertained up to endgame with new content. The AI system is better this time around. Monsters kill other monsters, and people fight each other in the distance (then you can go grab their loot after too). I encountered no major bugs.
There are more monsters, just the right amount to keep you guessing. No longer are they placed in predetermined spawns with only 2 in the whole game, like in Stalker 1. While obviously there are some premade spawns, they are more common and also randomly spawn I believe based on your progress in the game. One time I was exploring some area and randomly a fricken controller walked up behind me out of nowhere. Fortunately I remember there was one of those cloaky things there as well, and they attacked each other and the controller lost... lol. I also encountered a pack of burers in a field fairly close to the main base close to end game, certainly didn't expect them.
I think I have written enough if anyone even bothered reading all of it. I feel they got the gameplay very refined this time. They focus on the main point of the game and add elements that support them. Definitely worth playing, especially for those who gave up playing Clear Sky.