Ghost of the Grotto," the decision to lead with "Christmas on Bear Mountain," the tale that introduced Uncle Scrooge, is entirely understandable. I can even accept the decision to keep Scrooge off the cover, seeing as how this is a Donald-centric release.
Barks was still polishing his art and storytelling styles at this time. With the core cast still relatively small, the "ten-pagers" are primarily focused on the perennial theme of "Donald the hard-headed braggart." The hardy "rescue" trope that sustained most of the early Barks adventures is still being put to good use; for example, "The Ghost of the Grotto" kidnaps Dewey, while Donald, disguised as a kangaroo, is shanghaied by the bitter Mournful Mary in "Adventure Down Under." Ethnic stereotypes, such as the "wild bushmen" in "Adventure Down Under" and the sleepy-eyed Hispanic Volcanovians in "Volcano Valley," are frequently employed, though Barks is sufficiently wide-awake (see what I did there?) to upset lazy contemporary or modern assumptions about how he expects these stereotypes to be interpreted. (The bushmen are drawn in quasi-realistic, as opposed to "exaggerated minstrel," style, while the population of Volcanovia includes a somnolent Texan who had been "run out o' San Antone" and a couple of authority figures who seem eminently alert, at least when it comes to getting Donald and the boys in trouble.) But changes are coming; for example, we see the first signs of contemporary pop-culture references in stories like "The Waltz King," which pokes fun at classical waltz music, the "hepcat" pop music of the post-WWII period, and the cliched views that the devotees of each genre hold about one another.
"Ghost of the Grotto," in my view, marks an important turning point in the development of Barks' ability to craft a superior adventure tale. Ironically, Barks achieves this by keeping the Ducks in one geographical location for the vast majority of the time. The focus is on the gradual deepening of the mystery of the kelp-choked reef, the wrecked ship at its center, and the creatures who inhabit the place. The story develops logically and features numerous "mini-climaxes" (the Ducks crashing their boat onto the reef, the discovery and ultimate ejection of the giant octopus, Donald barely escaping getting blown up by the armored man's cannon) before a "grand" climax that, oddly enough, isn't much of one. By that time, we are far more interested in learning about "old jingle joints" and how he got to be the way he is than seeing the geezer destroyed, jailed, or anything quite so mundane. Barks probably overdoes the Olde English references a bit near "ye end," but one gets the distinct impression that he was in complete control of this narrative to an extent that he never had been before.