You may say it's not fair to compare them, but it's better to compare those than try to compare DCA to Disneyland -- DCA still comes up way, way short, and the only thing that makes it into a "full day" park with the addition of Cars Land, in my view, is that the lines at Cars Land take forever.
I'm actually not a big Harry Potter fan. I LIKE Harry Potter -- I read all the books, and I saw all but one of the movies, once each. I'm a much, much bigger fan of Disney. So, while I'd rank Cars as "less than" my interest in Harry Potter, neither one is particularly strong.
I was enchanted by the Wizarding World of Harry Potter. I thought the central ride was extraordinary, albeit too intense for many people I know (which makes it less successful, in my view) -- the queue area itself is just fantastic. There's something to see and look at everywhere you go, and one of the big "hits" for them is that even if you don't want to ride the ride, the castle itself is a major attraction and you can tour it on your own (and the tour is even slightly different than the queue line).
Each of the shops has a different theme, taken from the book and movies. But the wand shop sells wands. The magic shop sells tricks and gags. The candy shop sells candy. The Owl Post store sells little owls. It's not all the same "Universal Orlando" shirts everywhere you go. That's one of the BIGGEST misses throughout Disneyland at the moment ... but I digress. The stores at Cars Land are just, well, stores, and nothing made me want to go in them once I saw one. I wandered through the restaurant areas and looked at everything ... and then the feeling I had was, "What do I want to do next?" I didn't care about staying. I actually did want to stay and sip my butterbeer and take pictures of Hogwarts and even look at the cast members, who are so perfectly themed themselves.
I want to state again, I didn't DISLIKE Cars Land. I thought it was just fine. But I remember when Toontown opened in the early 1990s, there was a feeling like you wanted to explore everything, that it was really immersive and totally unlike anything you had seen. I didn't feel that at all about Cars Land, and when you get right down to it, with the exception of the ride-through portion, Radiator Springs is pretty much like a really, really smooth version of Big Thunder.
I know, there are a lot of people that will think I'm way off base, and that will think I'm being way too critical, that it's not fair to compare, etc. But now that I've been to Cars Land, when I think about going back to Disneyland and DCA, to be honest, other than Radiator Springs Racers, I don't really think there was much that left a major impression on me.
Plus, sadly, I think that in about 20 years, "Cars" is going to be seen as one of the more minor Pixar films, and I think it will fit alongside "The Sword in the Stone," "Meet the Robinsons" or "Make Mine Music" as one of the lesser of all Disney movies.