Part of it is the music. It’s funny but the opening scenes in OZ are a 15 mintue music dialog of singing. That has almost never been repeated in any other famous movie. Just a stringing along of songs.
Another sign it’s good is how much the audience draws that isn’t there. For instance, the Wicked Witch of The West is in the Wizard of Oz for less than 15 minutes in total, yet for some odd reason our mind thinks she is in the movie a lot more.
I agree with the OP that Gilda’s explination at the end made little sense, when Glinda says “She wouldn’t have believed me,” there was no prior evidence introduced to suggest Dorothy would not have believed her.
I also found it odd that when the Wicked Witch of the West appears she walks over to the house and says “Who killed my sister?” I mean if a house landed on my sister I would’ve assumed some kind of an accident. I mean who sees a house on top of someone and immediately thinks “murder.” I guess in OZ that must happen a lot.
I think a lot of people today insist that a movie has to be very real to have value. For instance, I’ve seen WWII movies that are terrific. They shove their message home, they are well acted, dramatic and just great movies, yet NO ONE SWEARS. OK we’re talking about a bunch of marines on Wake Island, you know in real life they were cussing up a storm, but those movies were so well written and acted you don’t even miss the fact no one is swearing.
Lastly Oz is a fun movie. It’s like Grease, we have Olivia Newton-John and Stockard Channing about ready to graduate high school and apply for social security. But we can overlook that 'cause it just that fun.
And the more fun a movie is the more of the minor trivial inconsistancies we can overlook