I would argue they're expecting Cars levels of merch, food, and attendance expansion. That said, unlike most here, I definitely think it could reach Potter levels of attendance. Especially with a good ad campaign. Imagine an ad with a family interacting with a hyper-realistic Na'vi animatronic along with images from the boat ride and E-ticket. That would sure get my kids excited! If you watch a few Avatar-related YouTube videos the comments are filled to the brim with "Wish I could visit Pandora"-esque comments. I think the demand to visit Pandora isn't even half as nostalgically high as Hogwarts, but will still be just as high due to the sheer levels of fascination of visiting Pandora, and experiencing what I'm told will be the best ride in Orlando, bar none.
Why do people say, "Pretty to look at, but otherwise...". As if it'll be uninspiring, or that the little details won't be there. I can tell you that this land will tell a story better than Diagon Alley, completely on its own. Disney and James Cameron have crafted a brand new story for this land, that has little in common with the movie. As for food, the overwhelmingly typical guest doesn't eat because they want to try some "crappy theme park food", they eat because they're hungry. Any new land, including HP hasn't/won't significantly boost food sales relative to attendance.
Consider this: would you rather spend your final day or two in Orlando paying $105 for a day at USF with the amazing Diagon Alley, or $68 /$60 for a 5th/6th day at WDW to experience the equally amazing, if not better, Pandora? This is Disney's target audience, and if more people choose the 2nd option, it could severely cut into Universal's attendance gains.