The English language is chock full of interesting oddities. I came across "kangaroo words" a while back; fascinating. Then Will Nediger one-upped that concept in an MGWCC guest puzzle; blew me away with his sheer genius! What's next?
There are well-documented variants of kangaroo words; several lists have been compiled. Most of the words aren't that interesting because they're short, easy to discover, or the antonyms aren't exact. I like the ones Alex picked, solid examples. ANIMOSITY to AMITY is my favorite because the letters in AMITY are spread throughout, much harder to discover than FEASTING to FASTING.
Intricate gridwork, rarely easy to execute on six themers. One might think that spacing them out, every other row, is optimal, but it can be extremely tricky. Stacking pairs can be easier, but there's something more visually pleasing about this every-other-row presentation.
Alex did amazing work in the middle of the puzzle, where problems usually arise. Not a drop of glue in sight. Having the flexibility to swap ANIMOSITY, COURTEOUS, EFFECTIVE, and WONDERFUL — as well as FEASTING and PRURIENT — gave him great latitude to try out different arrangements.
I wondered if EFFECTIVE would have been more effective where ANIMOSITY sits since the double-F can be easier to grid around when isolated. Here, it seems to have forced UPLIFT and thus YALU. It's not the worst offender, but the category of "constructor-friendly foreign rivers" gets a lot of eye-rolling from solvers.
As a musician, I loved the OCTAVE clue. It took me a few re-readings to understand what it was getting at, but singing "Some … where, over the rainbow" gave me a great a-ha, noting that both "some" and "where" are the same notes, one OCTAVE apart.
Great clue for GRAINY, too, referring to pics of the Yeti. Those touches go a long way toward making a solve playful and entertaining.