Little Mermaid is NOT a D ticket. It is pretty clearly categorized as an E up to the same level as Haunted Mansion. FLE is going to be a huge draw for Magic Kingdom, mostly because it's the first time in it's history that such a huge new area is added. The princess meet and greets as well as the Be Our Guest restaurant will be huge draws for the area but Little Mermaid is the pinnacle.
Still, do I think it will outdo Harry Potter? No, but it doesn't have to. All Disney has to do is add anything new and people will come, no matter what it is. Now that standard of quality may be a bit higher once people see how incredible Harry Potter is, but still, people will come to WDW no matter what, but adding new stuff is just another reason to come.
One thing I will give to Universal is that they are the best when it comes to building thrill rides. Mummy, Hulk, Spider-Man, Popeye and Bluto, MIB and Dudley Do-Right are all fantastic thrill rides and Forbidden Journey is sure to outdo them all. Disney makes some good ones, but the fact is Universal is better at immersion for their thrill rides.
Believe me, I love Everest, Test Track, the mountains, Mission: SPACE, Tower of Terror, Rockin' Roller Coaster and Dinosaur as much as the next guy, but Universal overall is better at building thrill rides. They will haul in the older families and teens while Disney will bring in the families with younger children for Fantasyland. That's one thing that Disney needs to work on a bit is adding more for older audiences to do.