I suppose that since the story was written by L Frank Baum in 1900, any symbolism may have been confined to that era? If you've ever contrasted with the 1925 version, I believe the MGM production machinery had much to do with the film's success.
The thing that strikes me about the movie, was that it was so revolutionary on so many fronts (like Star Wars in 1977):
1) Dorothy walking from Kansas sepia-tone to Oz's Technicolor REALLY WORKS!!!!!! OMG, what audiences in 1939 must've felt!
2) The lyrics by Yip Harburg are REALLY CLEVER (and beyond most any fantasy movie I can think of), the music by Harold Arlen was of course excellent. The Tin Pan Alley machine was REALLY happening at that time.
3) Of course, Bert Lahr (and to some degree Ray Bolger, Frank Morgan) have excellent comedic chops. Funny though, you see them in other films of the day and they're not nearly as funny. WC Fields was offered the Wizard part, probably good he didn't get it, he may have stolen too many scenes.
4) The sets, costumes, etc. are OUTSTANDING. MGM REALLY had it going at that time, but all that must've cost a fortune, even in its day the sets, costumes REALLY stood out.
5) Yes Dorothy and Hamilton are excellent, but Dorothy is kind of a modest ingenue/straight woman w a nice voice...it could've been Deanna Durbin or Shirley Temple? But Garland seemed to have an appealing underdog quality, a cutesy Temple would've ruined it. Hamilton was uniquely terrifying (and in other films and "Paul Lynde's Halloween Special", simply annoying.
6) Was this all a happy accident, or was there some George Lucas mastermind? I'm tempted to say this was an unheralded Mervyn LeRoy labor of love, or that Victor Fleming pulled a directing rabbit out of his hat. But Fleming was a kind of "man's man" director (also somehow directing "Gone with the Wind" simultaneously) and kind of gruff. How the heck did he do it? LeRoy once worked for Cecil B DeMille. Was the LeRoy vision to be as grand as DeMille, without the self-aggrandizing schmaltz?
7) Much credit to all the casting, sound, Munchkin trainers, key grips, etc etc, who pulled this together.
8) Like "Star Wars", truly unique/better on almost every front, a film for the ages!
9) Back to the possible allegorical implications, was this a foreshadowing/warning about fascism and the future Trump Presidency (ie "Pay no attention to the man behind the curtain!)
