So, Pirates of the Caribbean 2 does indeed have two separate, unrelated sequences where the heroes roll down the hill in a barrel. I'm not sure this was entirely necessary. (Spoilers ahead. Sort of.)
It invites comparisons to Star Wars, when taken as the second installment of a trilogy. The first movie was basically, self contained, because they don't know if they're going to be able to make a second. The second movie builds on what the first did, and ends with all these wacky cliffhangers. You know how the next one's going to start: they have to go rescue Han S--I mean, Captain Jack Sparrow. And you know who the big bad guy is, and why. (The heart, the death star--the details are different, but the basic message is the same.)
I still haven't quite wrapped my head around the structure. This thing has, at barest minimum, three separate dinguses. The heart, the compass, and Jack Sparrow. So you've got one people need because they want the other(s), one that starts out as two separate dinguses and then keeps transforming into different ones, and one that's actually a person, with all the wacky hijinks accompanying that trick.
That's a lot of dinguses. Add in the number of people after those dinguses, and just figuring who has what becomes an adventure in itself.
The other thing I noticed was there's just a lot of stuff in it. Stuff that only shows up for one sequence. (the cannibal/villager people never show up again) Stuff that is there mostly to make things even more complicated. (The jar of dirt being sort of a case in point. The only reason that item is there is so they can pull the three-way split.)
And then there's stuff that's just sort of there--like Davy Jones playing the organ. It's cool, and it adds a bit of complication to one scene, but they could have done the same thing with something else--something that didn't involve taking thirty seconds to show him playing it. But it's cool, because it links him up with the Phantom of the Opera and associated awesomeness.
The letters in the chest also stand out. They could easily have been taken out. They serve no mechanical purpose at all. But they're still really cool, because it's obvious what they are, and it underscores exactly what the heart is.
There's mostly just a lot going on. More than in your typical summer blockbuster. Maybe more than was in the original movie--I haven't watched it in a while. But I don't recall it having all the different characters and interlacing plots that this had. It did have several dinguses, though. And although it didn't have the amazing transforming dingus, it did pull a similar kind of sleight of hand with the whole "Ha ha! Not Turner! You fail!" thing.