This post is about some common newbie errors, but only ones not particular to cooperative multiplayer mode. I might write a separate post about those, later.
1. Getting the game's subgenre wrong.
Many players approach "7 Days to Die" as if it is a game about fighting and looting and finding an already existing building to occupy and fortifiy.
But that's not 7D2D's subgenre.
You don't have to fight all the Zombies you see. Many fights are avoidable. Sometimes, the Zombie you can see hasn't noticed you. Even in some cases when it has, you can run away from it and hide. It'll forget about you eventually. Especially early game, when you're underequipped and undersupplied, fighting Zombies is risky. Pick your fights. Don't be afraid to run away.
Likewise, you shouldn't rely on the RNG, Randon Number Generator, to give you the gear you need. Craft it yourself. Craft a Crossbow. Craft a Sleeping Bag. Craft a Stone Axe. Those are not optional! Same for food and drink; Don't rely on random finds of canned food and Bottled Water. You can't know that you'll find enough. Instead, make your own, grow your own crops (canned food won't improve Wellness at all, whereas most grown foods will), hunt beasts (cooked meat gives Wellness too) and brew your own Tea (you get Wellness from Goldenrod Tea as well).
Finally, existing buildings are poor places to live. They're decidedly suboptimal for the needs of a postapocalyptic survivor, and are hard to fortify. The proximity of other buildings also often causes strategic problems as those block LoS, meaning you don't notice approaching Hordes early. Build your own. Choose the right spot, then get to work. Making your own fortress is a lot more work than squatting in an existing building, but it's also a lot better.
I keep seeing, in YouTube videos, clueless players who make a beeline for the Central Hub City, to fight and loot, fight and loot, fight and loot. But while that is their intent, what actually happens is that they spend a lot of time dying or fleeing. They accomplish overwhelmingly little, in most such games. Sure, sometimes they get lucky, the RNG is "on their side" and they find the SMG or whatever. That doesn't mean it's the right way to play.
2. Failing to handle loot.
Even though 7D2D isn't a looting game, but a crafting game, you can and should loot.
With limited inventory space, you need to handle the loot you find.
You need to discard unimportant loot, e.g. Lead or Gunpowder, since you can easily find or craft a lot of those later in the game when you'll actually need them.
You need to craft and plonk down a Storage Chest, or several, to store items you don't need on day 1 or 2 but will need later.
You need to compact metallic loot (Brass and even more so Iron), by scrapping it, in such a way that the game's decimal number rounding process doesn't screw you over. Learn how the system works. 0.1 iron lost here, 0.1 iron lost there, another 0.1 iron lost later. It all adds up to that Shovel you won't be able to make.
3. Not understanding how Stamina works.
First of all, when your Stamina is low, the damage you do is reduced. This (probably) doesn't affect ranged weapons, but it does affect melee weapon use, and even more importantly it affects the damage you do when you "attack" voxels, e.g. when you're cutting down Trees or mining for ore or stone. Try to keep your Stamina above 40% to 50% at all times. As soon as it drops below that mark, take a break from your mining or logging. You'll get much more done that way, compared to if you work yourslf to exhaustion.
Secondly, I'm pretty sure that Stamina regain is non-linear. The lower your Stamina drops, the slower it regains. If I am right, then it'll be something like that if your Stamina drops from 100% to 70% then it'll take 15 seconds to regain, but if your Stamina drops from 100% to 40% then rather than taking 30 seconds, it'll take 35 seconds, or maybe even 40 seconds. Or even more!
I know it's easy to get carried away, when logging or especially when mining (for some reason it doesn't seem to happen much to me when I'm digging). You just sort of sink into a labour trance, and keep plugging away at the "enemy" voxel until you "kill" it, then you move on to the next one. But all that inefficiency adds up, so that you must perform more swings and spend more time to get the job done, compared to if you took more frequent pauses from the hard physical labour.
4. Not understanding how Tool Durability works.
Very closely related to the above. When a tool's Durability/Conditition stat drops low, probably to less than 50%, it starts to do less damage. This means you'll need more swings to destroy any given voxel, so it takes more time (7 or 8 swings instead of 6) and costs more Stamina,. All that inefficiency adds up.
Therefore, repair your tools.
Same goes for weapons. At least melee weapons, but probably also ranged weapons. Don't worry about being down to 85% ot 70% or the like. Don't repair obsessively. But do start to pay attention at around the 50% to 60% mark.
5. Not managing time.
At its core, any survival game, not just 7D2D, is about how you manage your time. You only have so many hours of daylight, each in-game day. What do you do? Do you spend enough time on day 1 on getting yourself a shelter for the night? Spend the next few days prepping for your first fortress?
Is your Crafting Canvas idle most of the time? Wood in particular takes a lot of time to process, from Trunks into Planks into WoodFrames, or from Trunks to WoodSpikes or from Trunks to Trunk Tips. Later in the game you have the whole Concrete Mix thing,and going from Iron Ingots to Rebar Frames. Lots of steps, lots of time. And you can't re-wind time.
For some players, of course, the problem is that they don't know that the crafting process continues even when they close their Inventory window and start moving about in the world, cuttin down Trees, looting POIs or fighting Zombies.
For other players, in coop multiplayer, the problem is that they don't know the interface and the recipes, and so they may very well be able to gather, and to fight, but they can't help contribute with the crafting, until they actively admit that they don't know and ask for help.