How much work must this theme have taken? After I finished the whole puzzle and finally figured out what was going on, I realized the challenges Mr. Chen faced in his construction:
1. Find pairs of phrases which share the same second word, and where one of the phrases' first word is one of "ten", "twenty", "thirty", or "forty".
2. Place the other phrase's first word in the appropriate slot in the grid which has the matching clue number from the first phrase.
3. Each one of those clues has to be a unique clue: that is, it can't be at a place where both the across and down clue share the same number.
All four actual theme answers are strong, as are the hidden theme answers. I love the [PET]ROCK turning into THIRTYROCK.
So there's a ton of work to begin with, but also note that two of the misplaced words cross theme answers, while a third is directly parallel to another. That puts a fair amount of constraint on the grid.
And the end result of that is you get areas like the SW corner, where ITZA, OWEN Wister (he wrote The Virginian, so he's definitely crossword worthy, but unknown to me) cross ZELDA and IOWA. That's a lot of proper nouns in a small area. Other corners fare better: the NE has SFPD (with a great clue) as well as classic crosswordese ERSE and NTH. In all, I count 19 answers that are proper nouns. That seems like a lot.
Still, there's room for IDLEHANDS and AIRSTRIKE, both solid answers. 1A: Joan nicknamed "The Godmother of Punk" (JETT) gets a strong B for the excellence of the personage invoked. I also was pleased with the pair of "What might get the ball rolling" clues (INCLINE and PUTT). Oh, and I was totally gotten by 27D: Capital of Sweden (KRONA) for the millionth time. Because it was a Thursday puzzle, I was convinced for a period of time that there had to be a rebus to account for fitting Stockholm into 5 spaces.
I definitely enjoyed this one.