I think that's because both use a type of "cell shading" it's the latest "popular" way to do some games. Wind Waker was like this and I didn't like it, S5 looks alot less childlike and cartoonish then that. Plus it's mostly the cut scenes that look this way, but look nice and refreshing from the usual 3d realism alot of shoot em games are doing. It almost feels like were watching Anime *g* The normal game mode seems an more updated 2d graphic look which gives a sense of nostalgia.
I can't wait to get my hands on this game *hehe*.