Here are some critics found when searching the web. I'm not always able to give the name of the author but most of the time there is at least one reference (magazine, date ...).
about Le Roi et l'Oiseau
What about the Studio Ghibli ?
Isao Takahata'opinion in an interview about the Grave of the Fireflies
My admiration towards Paul Grimault and "Le Roi et l'Oiseau" has always been the same, probably because he achieved better than anyone else a union between litterature and animation.
Hayao Miyasaki's opinion in the french paper Le Monde, 10th april 2002 (from CNDP)
Watching "Le Roi et l'Oiseau" by Paul Grimault taught me that space had to be used vertically. If you draw a village very precisely, it won't actually live if you don't introduce a vertical dimension. A movie needs a complete up-stroke so that the story takes on its real dimensions.