Difference is your family already know what attractions Potter and Carland have, and have probably seen reviews, photos and videos of the attractions on offer.The thing about this land is that it seems to be built on the idea that it's going to attract new clientele to the resort much like Potter did with Universal Orlando or Cars to DCA.
Is Avatar that kind of franchise? I don't know. But I know the first thing my family said when I told them Avatar was coming to DAK their only response was, "Why?" Whereas we've already been to WWoHP multiple times and have a Carsland trip planned for early next year.
I could see Avatar pulling more guests to DAK than say EPCOT, but is it going to pull a Carsland/Potter and attract people not already going to WDW? I'd be surprised if it did, but that remains to be seen.
If WDI can produce a killer attraction, I still believe that regardless of whether it is based on Avatar, or an entirely original story, it will be a huge hit. People will want to go and see it.
Reactions are weak now because there are no details on what to expect with the experience, and so people are just falling back to "I don't like the movie".