See, I "graduated" from the red box to AD&D 1e pretty late in 1e's run- I've never played the game without Unearthed Arcana! And even though 2e came out not long after, we continued to use a lot of AD&D stuff in our games- one of my DM's not only insists on using Comeliness, but he rolls Comeliness for his characters in other games, even when we tell him we're not using it!
OTOH, as near as I can tell, he uses Comeliness entirely as a shorthand for "how attractive this person is". When I applied the bonuses for high Charisma and modifiers for race for my characters, and started saying things like "NPC's of Wisdom less than 12 of the opposite gender are fascinated by me", his eyes crossed (I've never insisted on this point, since I have no way of knowing how Wise an NPC is, I just brought it up in passing).
I've played with 1e Rangers, 1e Monks, Cavaliers*, Paladin Cavaliers**, and a Barbarian once (and never again. Attack me for casting a cure wounds on you? Then stay at negative hit points, jerk!). Never a Thief-Acrobat (shame, they sound neat) or an Illusionist (given my experience with illusion spells, I'm not surprised why- it's especially bad that Wizards get a lot of illusions too!).
I've never used the abundance of polearms (my DM swears by his glaive-guisarme though), but I've long found "bohemian ear spoon" to be a lovely, humorous thing to say. But there were other things in UA that were never mentioned, and I didn't find out until recently, like the changes to multiclassing- there are some very neat combinations, like Halflings being able to be Druid/Thieves, that I'd love to have tried out!
Most of the magic items are silly, like Anything items or the Cyclocone, a Wizard hat that can create a tornado or somesuch- the only reason the Hat of Difference ever popped up at all was because it was in a published adventure!
At the end of the day, I understand why 1e fans didn't care for the book. It's on the same level as my Best of Dragon collections- it's a mixed bag, and it made changes to things people likely didn't want changed (see also Tasha's Cauldron of Everything). But for me, it's hard to talk about 1e without it, because I never experienced that version of AD&D. And if nothing else, the gonzo in that book prepared me well for the gonzo insanity of later 2e with flying ships shaped like squids, space nazi elves, Arcane selling wheel lock pistols in Waterdeep, devil people as PC's spouting Victorian gutterspeak, space bug renegade surgeons, double-specialized Elf Wizards, 10th level spells, characters with a page and a half of psionic wild talents, and the rise of multiclassed mythos priests with the abilities of every character class combined and more!
*Hate them.
**Hate them more.