Started replaying it again and the game doesn't really drag at all in its first chapters. You start off in a boat chase, flashback to the catholic school, fight in the prisons, search for the treasure in it, escape the prison, Drake and Elena's "normal" life, escape prison as Sam (hyped), and then pull a heist in Italy. I was engaged with it!
Haven't gotten there yet but I could see it dragging during Elena's chapter, but that has a lot of great payoff.
Then the platforming, it's engaging because movement feels
good. The prison has some great platforming sections, sliding, jumping, getting into fist fights with guards in between as you escape. You can't really die during these sections, nor does it achieve a Zelda like intricacy, but it's engaging anyway. I remember the marooned bit by the waterfall being a "mash X" heavy section of the game, but the traversal is mixed up more often than not by the rope, sliding, and jumping .
And as for the story being disappointing or just okay I can't agree. I haven't laughed as hard as Drake shooting a nerf gun reliving his past in the attic. The dynamic between Drake and his brother, Drake and Elena, the Goonies-like pirate adventure weaved through it. Its plot isn't remarkable but its execution is.
TLDR: After revisiting from some time away it's looking more and more like my GOTY, until TLG wrecks shop. And fresh off the TLOU (just revisited it) I think it's just as good, hitting me emotionally and damn fun to play. UC2, UC4, TLOU all masterpieces, take your pick.
Those downtime walking simulator sections are made fun by looking around, exploring for treasures and shit. The game is rich with detail and small hidden places.
UC4 has some of the best vistas I've ever seen. Libertalia is amazing to take in. Seeing the aftermath of the pirate battle with the cannonballs and dead skeletons laying in the courtyard, was a very cool "what happened to this place?" moment.
Can't wait for the Pro version of UC4.