I just beat this and I was thoroughly impressed. Surprisingly the game was pretty light on the gun fights. The had a few classic "run for your life" segments that were smooth and to the point. There was no confusing bullshit that makes you die repeatedly, and the hints are good again if you get lost, which I only needed about 2 times the entire play through. The majority of gameplay was cliffhanging and puzzles. They threw in some dialog choices at 2 points in the game, but that was it... it felt kind of forced, or maybe something that was scrapped and never finished, but they left it in those two spots.
The story was the best of the 4, and I liked that it didn't feature supernatural stuff. The journal was more like a scrap book where you kept folded up letters and clues with in it. It was very interactive, you could take stuff out and look at it when ever you wanted too. There was also a cool puzzle where you had to line up certain things and you had a few cut outs of it in your journal you could play with to match symbols. The letters that could be found, provided a lot of lore and went hand in hand with what you were doing. There were two stories going on at once, one about your bro, and the other about the adventure, they both progressed nicely through out the entire game.
The jump mechanics were very well polished, and the grappling hook was the best addition to the game. The game eases you into the added mechanics like the hook, gravel/mud sliding, and another climbing tool you get later through the game. eventually you have to string these all together in some with some quick thinking to get through areas. It definitely keep things interesting, rather than spamming the jump button from ledge to ledge, jutted out brick to brick.
The gun fights when they happened were pretty great. The variety of weapons was a lot larger, They toned down the grenade launcher to where you can't keep a lot of ammo for it (half the time it only had 3-4 rounds left when I got it), but the one shot kill weapons like the magnum and sniper rifles were more readily available than in previous games to where it was worth taking it to the next fight with you, rather than picking up a trusty AK with 150 rounds. The enemy types stayed exactly the same, but when you pick up the big chain gun, its actually useful, unlike it was in Uncharted 2. Provided it doesn't have 500 rounds and only 190... but at least they don't cheese you and make you drop it to climb up a ladder to the next section. Armored shotgun guys are still assholes, and I hate them.
In the end, I beat the game in 15 hours... It felt like double that though.