Finished the Uncharted 3 campaign last night (final playtime clocked in at just over eight hours), and I think I'm ready to call this one as "not as good as Uncharted 2". I don't think there are any spoilers in this; I've been as vague as possible and have avoided mentioning any locations or plot points wherever possible.
It's technically better than the previous game in a lot of ways, and the dialogue is just streets ahead of anything else I think I've ever played (although the plot is a bit too similar to, and it shares a few too many set-pieces with, the previous game), but some of the mechanics just don't feel right and the design doesn't feel as polished as I was expecting.
I'm also pretty sure that the combat in this game is garbage. As it progresses, more and more of the gunfight environments feel (as Minx pointed out) like multiplayer arenas, with so many paths for enemies to flank on that you can't just dig in to plan out your strategy, or even just to regain health. A lot of the areas seemed really dark to me, too - I'd often lose sight of the floor in the gloom, or run into an obstacle I couldn't see when trying to escape one of the armoured bastards later in the game.
Another issue I had with the environments was how huge they are; the ship graveyard in particular sticks out as the best example of this "problem". They've obviously been designed to look EPIC, but the downside is that it feels like it takes forever to get through them. Uncharted 2 had a lot of similarly-themed locations, but I always felt like I was progressing through them quite quickly. In Uncharted 3, I felt like I was in that shipyard forever.
Generally I thought the shooting mechanics were decent enough, apart from some of the bullet sponges you're up against in the final third of the game - but to make things a bit frustrating when shooting, aiming has an "assist" that slows the reticule down when you're close to an enemy. Which is fine against static bad guys, but slows it down so much that you can't catch up to a running target.
Coming from Arkham City really underlines how poor the hand-to-hand stuff is, too - I think Rocksteady have made the gold standard in fisticuffs, and it's a shame Naughty Dog didn't rip it off a bit more directly. The "counterattack" prompt in particular is shonky; several times I've hit the button while the icon is still on-screen but failed to block or dodge the attack that triggered it.
This seems like a big list of complaints, but I think I enjoyed more of the game than I didn't. There were a couple of sections that frustrated the hell out of me, but never to the point of rage-quitting. I think part of the problem, for me anyway, was the amount of time I spent with Uncharted 2 - the vast majority of my time with it was playing on lower difficulty levels with unlocked weapons and one-hit-kills, so maybe my expectations were wrong. I think being in basically the opposite position from Gar (I loved the last one, and had big expectations) might explain why I'm relatively disappointed.
I'm glad they sorted out the final boss fight, though. The first and second game both had, in my opinion, unfairly-designed showdowns with the Big Bad, but they've done something different with this one - and while it still felt a little unbalanced it wasn't anywhere near as frustrating as the Lazarovic fight at the end of U2.