SpANG!, you make a good point. How much plot can you get from an amusement park ride? (They added Jack Sparrow to the ride. That disappoints me. Greatly.)
Nonsense! You can get an amazing amount of plot from amusement park rides! Just look at the cinematic masterpiece that was The Country Bears!
I'm of two minds here. Basically, I felt that Pirates 2 should be subtitled "The Empire Strikes Back" because that's basically what it is-- a well done setup for another movie. There's nothing inherently wrong with the film, it just DOESN'T END. That's sort of irritating to me, because I have to pay $10 for a movie ticket these days, and I kind of want closure for that kind of money.
Well, the only reason people noticed the cliffhanger is because everyone was expecting a tight beginning-middle-and-ending movie package like the first movie. They filmed PotC 2 and 3 at roughly the same time (not unlike the two Matrix sequels, though they're still doing clean-up work on 3), and it was only natural for the two movies to complement each other.
Also it's important to remember that if you pay ten dollars for a good movie with a cliffhanger ending, it's likely you'll grudgingly shell out ten more dollars to see the rest of the story. You can go on and on about how a trilogy allows more narrative space and a deeper exploration of characters' psyches, but the fact of the matter is that Disney wants your money.
Besides, if you're so mad, why not write your own dang ending?