GENERAL GAMEPLAY
1) Fire damage and toxic sludge. In the worst case, a player can lose 2/3 of their HP from running into a few pixels of fire starting at the very first area. Of course water/flasks are an instant solution, but there are many times where these aren't immediately available. If a player does run into fire—which granted is often pretty fair and readable, but sometimes can feel pretty cheap—they shouldn't be at the mercy of level generation to save them half of their health bar. Similar to fire, damage from falling in an ankle-high puddle of sludge is pretty absurd when water cannot be found. Even only 1% exposure has taken away 20% of my HP. The most graceful solution would be to provide the player with an empty bottle at the start of each run. This way, a player can collect water (or other fluids) and prepare in advance. Alternatively, the "on fire" status effect shouldn't last quite as long, maybe even ticking a little faster to compensate, and toxic sludge should tick for less if it's going to last as long as it does. Perhaps sludge ticks for a lot less at low values, but does significant damage if the player is fully drenched? Consider making sludge glow a little for better readability.
2) Taking damage disables flight. I think it's completely fine to briefly lose your ability to fly after taking damage. However, being unable to move completely until you hit the ground, regardless of how far you fall and how much damage you've taken, is extraordinarily frustrating. I've had many instances of flying over a large pit, getting hit by a tiny projectile that dealt 4 damage, and the hurtling down into instant death in a variety of different forms. Taking damage should not stunlock your character into oblivion, especially since it's already so easy to die. I'd prefer if very small sources of damage, say <5 or so, do not cause any form of cc on the player, moderate amounts of damage disable flight for a very brief time (<0.2s) and only large sources of damage displace a character and disable flight for a little longer. I believe the player should never need to run into the ground after taking damage before being able to fly again.
3) Worms breaking things. I'm not quite sure how/when worms are programmed to spawn, but every once in awhile they'll seem to appear out of nowhere while I'm inside the holy mountain and destroy everything. This apparently pisses off the gods, and the worm ends up passing on the blame to the player instead! Random events out of the player's control shouldn't immediately summon a bone god and incinerate the player. If possible, the holy mountain should be invulnerable to monsters, or at least the game should realize it's not your fault when worms and spiders break things.
4) HP Upgrades. I have mixed feelings on the way HP pickups are handled. I'm ok with Noita's Spelunky-style system where one mistake can get you killed—especially when it's obviously your fault. This mechanic can make finding health upgrades that much more rewarding. However, even Spelunky gives you a consistent way to increase your health pool without long time commitments. Want health? Just listen for the damsel while traversing the level. Noita makes finding health either really difficult with unlucky world generation, or incredibly easy on certain seeds. Part of the problem is that there's no direct indication of a pickup's presence until you get close, but also because each level is so damn big! I could spend 10 minutes searching around for an HP upgrade, and even when I do, it might be obstructed by terrain. Health upgrades should be a little more clear in a level, either by emitting light through walls, playing a little beep when you get close, or even something more creative like a little pixel trails signaling its existence over long distances (like the particles near a teleporter in Risk of Rain 2).
5) Enemies can fire any wand they pick up. This is very problematic for wands that are absurdly destructive. Combined with perfect enemy accuracy, dying to an off-screen nuke is funny at first, but on runs where you, y'know, actually try to win by spending an hour+ building spells and collecting orbs, dying to a single purple death orb while at 160 HP with stainless armor makes me question if avoiding death comes down to luck in most cases. I'm not sure how to handle this one, but maybe disallow enemies from firing certain spells like nukes and other spells that can kill the player in a single hit? Enemies picking up wands is a cool mechanic, but just needs to be tuned for a few spells.
PERKS/SHOP ITEMS
6) Fire/sludge immunity. Tying in with my points on fire and sludge, it's ridiculous how useful fire/poison/explosion resist perks are, probably a lot more than they should be. It might just be me, but most of my deaths occur directly from passively burning to death from the environment or dying nearly instantly to an explosion or missile from the darkness. I'd prefer if fire/sludge were to be tuned down a bit, then you could possibly even remove straight immunity perks in favor of resistance perks. Say, perks that instead make fire/sludge do quarter damage, wear off quickly, etc.
7) Immunities in general. Being immune to a specific damage type is extremely useful, but often too much so. Oftentimes they are extremely situational (like melee resist, who's crazy enough to get that close?) or just straight out too good (see point above). How about perks that are slightly more general but still tie into the themes of each immunity upgrade? Instead of straight explosion immunity, how about heavy armor that limits incoming damage to 25% of player HP for any one attack? Or light armor that reduces that damage of each incoming attack by 2, rendering fast attacks weaker but not impacting heavy-hitting attacks? Electricity immunity is a weird one, because it's already not a common damage source, yet there are two perks that do the same thing, but with one also creating electricity around the player. Why is this the case? Why not just remove the pure electricity immunity if there is a far more interesting choice that also has some negatives to balance out how good it can be (like accidentally exploding metal barrels)?
8) Stainless Armor. Because of how the the status effect system works, stainless armor can lead to a pretty infuriating cycle by itself. Killing an enemy and having fluids spattered all over you immediately negates the effect of this upgrade for quite awhile. Did these fluids happen to be sludge or something involving fire? Sure you could jump in water, but then you need to wait for all of that water to dry off before the upgrade kicks in again. Conversely, perks like the small shield can negate a ton of projectile damage without meeting any specific criteria, making it a strictly superior upgrade on most occasions. One fix would be to keep stainless armor active if the player is simply wet. Also, it would be nice if stainless armor granted some sort of visual indicator that it's actually working.
9) Teleportisis. This perk should not interact with status effects. While it's fun the first time to teleport everywhere when covered in toxic sludge, it usually guarantees the end of a run since I often just teleport into more sludge, or really anything else that just kills me or teleports me into more death.
10) Vampirism. As far as I can tell, this perk is the only way to regain health during a floor currently. Unfortunately, it feels a little lackluster since you need to drink a lot of blood to gain back a useful amount of health. For reference, an entire blood cloud only heals around 15 health. Combined with losing 1/3 of your max HP, this perk rarely feels good compared to others. If all else were to remain the same, remove the part that lowers max HP. Alternately, increase health gain, or even better, make the perk do something a little more interesting. Perhaps you automatically absorb nearby blood for HP? Maybe being covered in blood makes you even more powerful?
11) Perk Lottery. I absolutely love this perk, but god can it be pretty broken. There's hardly any risk whatsoever, and if this perk is found after the first level, the player will on average gain access to 4ish additional perks by the end of a run. I think odds of finding an additional perk could be fine-tuned, potentially with a chance of getting no perk at all when you pick one up!
12) Edit Wands Everywhere. Sometimes I wish you could do this from the start, but I also appreciate being forced to traverse an entire floor with random wands before dismantling and improving them. I think this should be less of a perk and more of an upgrade. The way to obtain it, however, is open to discussion. Buy it in the shop? Unlock it over multiple playthroughs? Some wands always allow it while others don't?
13) Shop items with more detailed descriptions. Spells in shops should tell you what they do when nearby. Of course I could open the "progress" tab, but it would save a ton of time with those descriptions innately above their respective spells.
14) Oil/Poison/Other "trail" spell effects. Don't get me wrong, these spell effects are cool and provide the player with some interesting combinations, but more often than not they are extremely useless unless paired with their respective immunity perk. Usually I'll end up burning to death or suffocating when using most of these. I don't quite want to see these spell effects go, but how about changing the way they work? Maybe after a spell ends, it explodes into an element. Really just any way to keep the resulting fluids away from the player. Poison trail is nearly guaranteed to hurt the player more than the intended enemy, and I just avoid it at all costs now.
15) No place to sink gold. Currently, I often find myself in situations where the wands I find are better (or at least good enough) compared to shopping for wands or spells, meaning that I'm forming a habit of ignoring most gold, especially if it's covered in lava or sludge from the enemy that dropped it. Some more ways to spend gold would be nice, perhaps on health/flight/wand upgrades.