I only know what my problems with the show are, but they include:
-Book issues galore.
-Direction issues (tying in with the book issues)... why is the Dragon never really explained? How is it relevant in the show? If I hadn't read the book I'd be confused.
-Clockworks are so significant in the design, but never really explained WHY clocks are so important in Oz.
-Score. I love some Schwartz but this score makes me sad. there are a handful of good songs, and clever lyrics, but most are phoned in and banal. Defying Gravity is really an over written, and over sung most of the time, song. Something Bad is just... well, bad. The rest of the score doesn't do much for me. I do like Popular, I'm Not That Girl, and For Good. But that's just my opinion.
-Why at the end of Defying Gravity does she just lift and SIT THERE! It's so uninspiring. She should be flying all over the place like they did in EASTWICK.
-Final Act One lyrics: Look at her she's wicked, GET HER! No one mourns the Wicked, so we've got to bring her down. Schwartz has gone out of his way to make up cute and quirky lyrics that RHYME through the whole show, then we get this awful moment of shouted lyrics that don't rhyme and just sound awful.
And yes, it's over-produced.
Why it's a huge hit, I don't know. I suppose it's easily accessible because of its original source material. But I still think it's poorly written, and staged.
Mind you, that's only my opinion.