I really agree with your point about cards that are banned in some formats not necessarily being mistakes. I think there were a lot of genuine mess-ups in 2019, but some of the stuff that I see people deride as huge mistakes are cards that I think are great. Veil of Summer is a good example--too strong for standard I guess, but I think it's a good addition to Magic as a whole and I'm glad it wasn't held back by Standard-centric design.
@evouga The snow lands are actually priced... well I won't say "reasonably," but competitively at least, relative to the cheapest available foil snow basics. The issue there is that you get one of each which makes it really expensive to get the ones you need for a deck or two, assuming you don't actually need even numbers of all five.
The cat one is definitely the odd one out and seems to be more testing the waters for off-the-wall art in special editions, more like the SDCC promos than the rest of the secret lairs.
Leagues are where you just play five matches on your own schedule and get prizes based on your record. You can ignore the start/end dates... that's basically just when the leaderboard resets I believe. It's always running.
Challenges and playoffs are actual tournaments, one of which happens every Saturday. Challenges are open, playoffs you qualify for through challenges and leagues. Playoffs then qualify you for an annual championship tournament, iirc.