Okay, let me give my two cents in a non-hyperbolic way. I'll try to break it down as good as I can...
Ikoria feels like there simply aren't enough different elements that make up the world. It's just shallow and incredibly symmetrical across all five Wedges. The triomes look and feel way too similar to me, and what good is establishing a dominant clade of monsters or an apex predator for each triome when you're just randomly mixing an matching creature types anyway? There are just way too many creatures that just look like random amalgamations that you can't identify without looking at the type line, and even then, what kind of identity does a monster with three random creature types even have? I already didn't like the direction they were going in with the Simic the last time we saw Ravnica, but Ikoria takes it to an extreme. Creative combinations of creature types can be cool, but they are cool when they stand out, not when the whole plane looks like that.
I would have preferred it if more of the monsters had had a clearly identifiable and visually appealing species. And maybe they could have done more with known Magic monsters that already feel like hybrids or could have a more modular anatomy, like hydras with different kinds of heads, chimaeras, cockatrices, basilisks that have been merged with other reptilian creatures, pegasus-unicorns, that sort of thing. The beast in Vivien's artwork looks pretty cool actually, something along those lines would have been way more appealing to me than sharks with arms or Dinosaur Hippos. Speaking of that Dinosaur Hippo, what is it with the tone and artstyle of this set? Between Battlebond, Eldraine and Ikoria, why are there so many products in Magic that look like they were made for children? Not everything has to be dark and gritty, not at all, but they keep moving the aesthetics of the game in a direction that sometimes feels closer to an Un-set than to regular Magic. I mean, the genre or source material that inspired Ikoria just isn't something I believe works well in Magic, especially not as a basis for worldbuilding.
The fact that there are basically ONLY monsters and humans adds to the blandness. Sure, Zendikar does the "characters vs environment" thing too, but at least Zendikar has a variety of different races that feel disitnct and interesting in their own right. Ikoria only has humans, which is clearly an unfortunate side effect of the Mutate mechanic. And no, I don't care that they live in different cities and wear different coloured crystals on their costumes, they're all human, and they all have a generic "adventure" look (and even that was done better on Zendikar). The fact that some humans stay at home while others venture out to kill monsters (only to be magligned as "the worst kind of humans" by a planeswalker protagonist I can't stand) isn't exactly grounbreaking either, and neither is the fact that some of them are Pokémon trainers. Like, seriously, this "bonding with beasts" thing has been done a million times, both in Magic and elsewhere. The most generic of them all in every way imaginable is Lukka, which is perhaps fitting because like many planeswalkers, he is supposed to be a perfect representation of his homeplane. Just wait until they establish he can actually planeswalk with his pet cat, because I'm telling you now, it's going to happen...
The Companion mechanic feels super cheesy and out of place in a regular Magic set. Seriously, we have Commander, Brawl, Tiny Leaders and Oathbreaker, why the heck do we need this random mechanic to blurr the line between the feel of different formats even more? If they wanted to make use of deckbuilding restrictions, why didn't they reserve that for the Commanders in the decks and word it like "If [cardname] is your commander, your starting deck [insert restriciton]". Muate is just kinda messy and unimaginative at the same time. I think there are going to be a ton of corner cases with cards that interact with it in ways that make it hard to decide what is going to happen. And I think a pile of Mutate cards on top of each other with a random assortment of keyword counters is going to create more memory issues than it's worth in terms of fun gameplay. I realise that "build your own monsters" is the selling point of the set, but to me that sounds like a giant mess.
And boy, do I hate those Godzilla series cards. It's just sad that Magic has arrived at a point where it has to pretend to be something else in order to sell packs. That's not even mentioning all the other issues that I have with special collectors bling alternate versions of cards and the way they are getting out of hand...
Edit: Raven ninja'd me, but yeah, basically what he said.
WAIT, Raven is back??? Wooooh, I really missed you around here Raven, great to see you!