From a person who has played every core series game, here are just a few negatives about Pokemon Legends: Arceus:
Tutorial - too long and linear
Battles - unbalanced, and yet not at all intimidating
Open world - not truly open world
Dialogue - too much, and repetitive. I don't need to be told the same thing multiple times in one (or more) interaction(s)
Voice - needed voice acting
Water - can't swim, which just seems ridiculous. If anything, add a stamina bar, since stamina is already implemented
Camera - wonky, and at times hard to control
UI - extremely difficult to adapt to, far too many menus located in different places.
Graphics - should have been far better, lots of glitches, and so monotonous
Difficulty - geared towards children, holds your hand every step of the way
Puzzles - no challenge, almost always told exactly how to solve it
Side quests - repetitive and simple, and then some that are insanely tedious and not given good enough direction, side quests list should always show markers on map. New requests should also appear on main map. Does not need a menu to select one at a time. Order is always changing
True to core games - missing anything resembling gym battles
Story - good idea, but not polished, and becomes very boring quickly
Wild pokemon - seem over-powered for their level.
Fast travel - should be able to fast travel from one area to another once an area is discovered, especially back to the home village
Stamina - incredibly annoying and unnecessary
satchel - gaining new slots should not cost a million.
Stealth - many times Pokemon can still see you even when crouched in the grass, which is supposed to hide you completely
Crouching - while crouched next to rocks, mounds, or other objects makes you glitch and then Pokemon will spot you
Markers - should show up in real world, and should have multiple flags
Price - far too much, should be at least half
Pokedex - navigating to find specific pokemon can be a real pain, because of the different areas. Just solely have a main Pokedex instead of multiples showing Pokemon only in specific areas
Eye Indicator - indicator showing Pokemon have spotted you doesn't always appear, and doesn't always disappear
Spawns - Pokemon (including alpha) can spawn right on top of you, especially after certain cutscenes
Attacks - some moves (i.e. bite and crunch) seem to do the same damage, even if the attack style is the same
Crowding - some areas (i.e. scarlet bog) have way too many Pokemon in such a small area and all of them are attacking, making it nearly impossible to walk through, while other areas are completely barren.
Controls - should have an option to turn off vibration, and also button customization. Switching between pokeballs and Pokemon is always frustrating.
Pokemon speed - should have the ability to see the action order before opposing Pokemon attack. It's unfair to get hit twice at the beginning of a battle. Pick a move and then let the opposing Pokemon attack
Shops - everything is far too expensive, also new items in stock (from clothier for example) should be indicated in the list.
Character customizations - have more hair styles, more clothes, same options for male and female
Lost items - should get more MP for retrieving, otherwise it doesn't seem worth the time gathering other players' satchels
Charms - should have charm slots so they can be equipped and able to use as long as it is equipped
Pokemon research - good concept, but way too much work to get 100% on each entry
Ride Pokemon - should be able to throw Pokemon and items while riding and sometimes for no apparent reason, you cannot get on your ride Pokemon