I had to think about this for awhile, because while I do vastly enjoy this game for a variety of reasons, there is one major aspect that has continued to frustrate, only getting worse as time goes on.
First, let's start with the positives.
Adventuring and exploring is great, progression is delightful, and there are so many enjoyable easter eggs, and a decent variety of ways to play through the game itself. The coop experience is wonderful as well, and even though not everyone is able to get some of the legendary items due to limited availability in the world, the rest of the available gear is still comparable enough to make this a more minor issue. If you are a fan of Terraria or other similar survival/adventure games, then Core Keeper will likely be right up your alley.
Now for the one major issue that I have with the game: the leveling system.
I love leveling systems and talent trees, and while the talent trees in Core Keeper are relatively simple, they are nice. I would certainly love a more in-depth talent tree, or even some of the less-useful talents reworked, but that is not where my main complaint lies. My main complaint lies in the experience system specifically. See, most games follow a general pattern for gaining experience. Do a thing related to your skill, and you gain experience in that skill. The harder the thing, the more experience you gain. For example, slaying a boss would grant you more experience than slaying a minion, or a common mob. Crafting a complex and/or expensive item gives more experience than crafting a torch. Fishing up a tricky fish awards you with more experience than pulling up a sleepy, never-moving fish. Core Keeper follows this principle for some of their skills.
See, for a lot of the skills in Core Keeper, you simply gain 1 xp every time you do something related to that skill, no matter how difficult or easy it was. Out of the 9 skills available in Core Keeper, 7/9 of them follow this rule. For some of them, such as Cooking, Gardening and running, this seems to work just fine. Maybe not ideal, but fine. For the rest, this rule ranges from annoying, to downright aggravating.
On the simply annoying side, we have Fishing. While Fishing, you can either pull up a fish and start the fishing minigame, or you can pull up an item. In either case, you gain 1 xp. It doesn't matter if you pull up an easy fish in the beginning biome, or an epic fish from one of the lategame biomes. It doesn't matter if you pull up a piece of scrap, or if you pull up a piece of rare gear. It is all 1 xp. This leads to the strange conclusion that in order to level up fishing, you don't want to fish up fish. The fishing minigame takes more time than simply fishing up an item, and nets you the same xp, so if you want to level up fishing, it's better to fish up items, not fish! I am not a fishing fanatic, so this mostly is just annoying, but for anyone who really enjoys fishing in various games, I could see this being quite frustrating. (Also, what is the point of the talent that makes you fishing rod casts go further?? I could see this making it easier to fish from fishing shoals, but you can also build a dock easily enough to make this negligible.)
For the frustrating, we have Ranged, Melee, and Mining. For all three of these, you get 1 xp each time you hit something. For Ranged and Melee, it's 1 xp for hitting a creature with a ranged or melee weapon, respectively. For Mining, it's when you hit a wall and deal at least 12 damage. Like most games with leveling, the higher the level, the more xp it takes to get to the next level. These xp requirements become egregious at the higher levels, especially if you do not understand the xp mechanic in the game. I spent the entirety of the game with a melee focused build, and even beat a couple of the end game bosses, including the Atlantean Worm. By this point, I was barely over level 50 (out of 100, for perspective). For further perspective, the xp required to get from level 1-50 is approximately one quarter of the xp needed to get from levels 51-100. I had not touched a single ranged weapon this game, yet I was at end end game and I was barely 1/5 of the way towards 'mastering' melee.
It was around this time when I started scouring the wiki, and learned more about the various mechanics of the game, including the xp mechanics for the various skills. Like many others, I love gathering achievements and I wanted to find the most efficient way to get those delicious level 100 achievements. Imagine my disappointment when I realized that the best way to level up Melee was to fight an early game boss with my fists... over and over again. Not exactly exciting gameplay, and most definitely not intuitive for the majority of players. However with over 15 Hive Mothers slain by my fists, I had made decent progress on Melee, getting to about level 90. They last few levels are truly a slog, taking at least 1 boss fight each to complete. So leveling Melee proved to be viable, even if it was a bit of a slog. Though, at the rate that these last few levels are taking, I'm estimating at least 10-15 more fights before I get level 100 Melee...Ranged is not much better. In fact, you could argue that it is worse, since you have to use a ranged weapon, meaning that you kill enemies in fewer hits, which nets you less xp... and you have to contend with durability.
The one that I'm dreading the most is Mining. In games like Core Keeper, Terraria, Minecraft, and so on, Mining is a fundamental aspect of the game. You mine constantly, breaking down walls to get to ores, to explore new areas, to build bases, to make travel easier, and so on. I love mining for ores, I love spelunking into the depths and exploring caverns. With over a decade's worth of experience of mining in Minecraft, my love for it still has not diminished. However, the process that will be required to even attempt to bring my Mining up to 100 in Core Keeper... is asinine. If you are playing like a normal player, you don't want to spend forever destroying each individual wall, so you increase your mining damage to destroy each wall in 1-2 hits. If you do this, you would have to destroy over one hundred thousand walls individually to get level 100. You can't even speed up the process by using hammers to destroy 6 walls at a time, because that still only gives you 1 xp, because it is '1 hit'. To help make sure that your world doesn't look like a strip mine, it's better to level up your mining by hitting one wall several times. Except, if your mining damage is too low, you don't deal damage to the wall. If you don't deal at least 12 damage to the wall after it's armor, you don't get any xp at all. You can try trial and error to attempt to find the 'optimal' walls to grind xp, but it's incredibly tedious. Your best way to do this is to once again use the wiki to find the exact stats for each wall, and then use that info to find the best wall to punch over.. and over... and over.... My mining is level 72, but that is just barely 1/4 of the way to level 100.
Vitality and Crafting are the only two skills that do not follow this rule, with Vitality giving you xp based off of the max HP of the monster you killed (capped at 1000...), and crafting giving you more xp for the more complicated/valuable items that you craft. I think that Vitality could benefit from having the cap removed and the xp adjusted for later levels, but honestly, it works well enough as is and probably does need any adjustment. Crafting works well enough, so no further notes there.
This review turned out a lot more long-winded and technical than I initially expected, so I'll try and summarize here. I simply ask that the xp system gets reworked for at least a few of the skills. I am not asking for gaining xp in various skills to be trivial, I am simply asking for it to be feasible. To be enjoyable.