I'm with this guy.
Here are the things I use to try to keep the game genuinely fantastical...
1-In a sandbox, knowledge is freedom , so none of this means players should be enmeshed in such a mystery-universe that they can't make decisions. Only that a certain level of inexplicability needs to be enforced.
2-In the process of trying to get shit to work and trying to make jokes, players will do their damnedest to banalify the gameworld. Usually accidentally. That;s ok, it's their job. Just as you keep trying to kill them you have to keep trying to surprise them.
3-You can go completely the other way and still be a great GM. I've seen it done. Pretty much every time I've been a player. This is just my rules for me.
4-Most of this is probably old news to longtime readers.
So here's what I try to do:
-A player who has never played before needs to see a picture or a mini or they need to be able to imagine the creature just from the words you use to describe it.
-The word should be evocative in itself. "Goblin" is an evocative word. "Kobold" isn't.
-Don't use the same monster twice--unless there's some clear connection to the first time it showed up. Like goblins can show up more than once because it's established there's a goblin empire, but that wandering monster table with giant spiders on it again has got to go.
-Never use a monster with the same basic visual idea twice unless there's some clear connection to the first one--like never use 2 spider monsters, 2 two-headed monsters, 2 snake monsters, 2 wolf monsters, etc.
-Each session should include at least one new, freakish thing that you made up yourself.
-No grass. It isn't there. There's snow and, under that, stone. Elsewhere there's water, or sand. Maybe there's sand in the Sythian plain far off to the east, maybe. There might be cows and/or farmers, but you never hear about them unless you, the PC, specifically seek them out.
-Never make monsters "scalable" unless they're basically humanoids with levels like the PCs and so the PCs can tell them apart as characters. Dragons are big and tough and there's no ascending-HD parade of firedrakes or fire lizards before you see dragons. Your first dragon is a scary big deal. And maybe also your last dragon.
-Using the word "dragons" plural, is a warning sign.
-No standard-D&D alignment-as-cosmology.
-No ubiquitous public organizations. "Society of the Back Ear" or whatever.
-Finding an NPC cleric willing to heal you is fucking hard and generally involves some creepy religious thing happening. Because miracles are rare.
-Don't write down any detail of the game world unless you're having fun writing it down.
-Don't write down any worldbuilding detail unless the players could: A) Find it out, and B) Care if they found it out.
-In other words, try to treat building and revealing the world as much like a fiction-writer as possible while still revealing enough to allow meaningful player choices all the time.
-It sure helps to have players who have only been playing for a few years.
-There aren't very many magic items you can carry and use--and the ones you do find usually just work once. You'd never find a "sword +1".
-It's hard to figure out what magic items do.
-If it looks dumb in my head, I'm not putting it in the game. No fucking running around whomping on an owlbear with a fucking Ioun Stone whipping around your head. The players will make things silly all on their own.
-No self-referential "screwjob" monsters: Oh-you-thought-it-was-a-rust-monster-but-really-it's... Trickery is fine. Trickery that assumes the players think the world's a bunch of tropes strung together isn't.
-You don't know what the spells cast by the enemy wizards are called. And if you do, it isn't "improved invisibility".
-Never have an NPC use the same spell twice. At least not in the same way.
-Try to give PCs access to nonstandard spells at every level.
-Adventuring is weird. Never use the "thousands of adventurers have been in this dungeon before" trope.
-Players have an understanding of the world that is imperfect, but accurate enough that they know basically which subgenre is in each direction. Even when the provided maps are basically 100% accurate (down to inch-per-mile-scale), they look inaccurate and eccentric, like a mappa mundi.
-No "chain of humanoid enemies". Goblins are weird fairy tale monsters with their own empire, gnolls are slavering barbarians, jackalmen wear robes and know magic, crowmen are semidemonic and rare, white leopardmen serve a Frazettastyle ice witch and bugbears and hobgoblins and what-all are bizarre unique things you haven't met yet.