Tiger926: I suspect it isn't a Signature restaurant due to the ambiance--or rather, lack thereof. Disney describes it as a casual and lively cafe. Most of the "lively" comes from the fact that it dosen't have an "entrance" per se and has no front wall . Anyone sitting at one of the front tables can actually shake hands with people strolling by. And has been noted several times, it is noisy, from the lobby below as well as the crowds at nearby Ohana and the Tambu Lounge.
None of which should suprise because for looks, it is mostly a reincarnation of the old Coral Isle Cafe.
Which is why we've never had dinner there. We always eat breakfast there each trip, but it is way too noisy and "lively" for us to enjoy dinner.