Production IG has managed to achieve is a show that truly looks shoujo in a way most anime have not, including perfectly good works such as Cardcaptor Sakura and Itazura na Kiss.
There’s a very strong understanding by the staff on this show as to when to utilize the more visually two-dimensional and emotional elements and when to incorporate a three-dimensional sense of spacing among the characters, and it, and the result is that it achieves that pastel and wispy shoujo manga feel while still making sense as an animation. In a way it reminds me of 70s-style shoujo, only with less emphasis on the melodramatic, and more technical skill on the part of the animators.
I think it's a good time to remind everyone about another P.IG production with a similarly singular visual style, Windy Tales [link]. There's even a small overlap in staff, for example Masatsuguru Arakawa worked on both.