I'm not sure that everyone has given the plot of World's End if they thought it was easy to understand, let alone claim that a 10 year-old could understand it. Perhaps its more that I just didn't believe it - a lot of the dialogue in the film seemed to be the characters explaining the plot to us - and the explanations were almost as convoluted as comic book continuity (and if you've read as many as I have, you'll understand what I mean!). Probably the aspect that I had most trouble understanding was each characters motives: why they did what they did.
Particularly in the Jack Sparrow scenes, Pirates 3 seemed like a movie that knew it was a movie...
But for all my criticisms, let me re-iterate that I thought this was a good film.