I'm totally out of the loop on this Potter craze. I've seen snippets of one of the movies I think, (British kid in dorky glasses with wizard powers) but I don't know anything about the franchise and it just doesn't interest me. And junior high kids like reading the books? Or something like that? If it has kids reading, it must have merit.
That said, I find it hard to believe the opening of some of the land without any new rides will have any impact on Disney attendance. I'm sure it will be a nice thing to stroll through for people who were already going to Universal, or it may drive some attendance from Potter fans who otherwise wouldn't have been in Orlando. But it won't drain away any measurable attendance from the WDW parks.
I liken it to New Orleans Square opening in 1966, and Pirates of the Caribbean opening in 1967. Walt had a fun little dedication ceremony with the then mayor of 1966 New Orleans in July '66. But since Pirates wasn't ready yet, it was just some shops and restaurants except the Blue Bayou. Pirates of the Caribbean finally opened in March, 1967 with the Blue Bayou as well, and that's when attendance went through the roof and everyone was talking about the fantastic new pirates ride at Disneyland.
It seems Universal Orlando is about to do the same thing by opening up a newly built land of shops and restaurants, with the big new ride still behind walls and not opening for another 9 months.