What was the age of Fuse when he was writing Tensei Shitara Slime Datta Ken(WN)?

I won't be so quick to judge the authors for that. Japanese language plays a part. Given that the translations are done without the author's input, then it is not a surprise.
Light novels should do a better job. I would believe 

By "Show, don't tell" I meant this, namely
"Show, don't tell is a technique used in various kinds of texts to allow the reader to experience the story through action, words, thoughts, senses, and feelings rather than through the author's exposition, summarization, and description. It avoids adjectives describing the author's analysis, but instead describes the scene in such a way that the reader can draw his or her own conclusions."​
However, when searching for an example of the novel lacking in this regard, I understood that "Show, don't tell" is not even the main problem. The lack of exposition (Volume 4, the moment piercing the chest of a certain character) and the 'First person' POV writing style.

Dear Jigoku Shounen, I was not talking about Foreshadowing at all.
More than this, I do not even have any problems with Foreshadowing.

What makes me sad is that these novels are not fun to read for me. Asian novels have interesting concepts, but are lacking in following the basics of writing.
We have examples of the authors that followed the basics (Reverend Insanity(1st vol), Mushoku Tensei, Dungeon Defense, Vermillion) and those who did not (Ouroboros Records, this novel, plethora of other novels). The difference in levels of entertainment is huge.


So, ok, overall I understood that Fuse didn't reveal his age.
Therefore, I have the last question to Jigoku Shounen. Am I still the newbie reader? If so, could you please elaborate on the "Most Common" writing?
Because if the majority of novels are like this one, I have no hope of finding something reasonably entertaining for me anymore. 

Yeah, there is a lot inferred in Japanese.

I personally like the fact that most web novels don’t take 6 pages to describe what a feast looks like. I also don’t need a 2 page description of what a forest or castle looks like. 

There are no real rules in writing. Any writing book that tells you so is lying. There are things that most people like, but every single rule has been broken by some classic author.

And that doesn't even account for the fact that different groups, especially cultures, consume entertainment differently.

Edit: There's no reason to apologize for not liking a writing style. Perhaps you just aren't the audience for that writer. 

I am just too frustrated from the breaking of the "Show, don't tell" rule in Tensei Shitara Slime Datta Ken.

Therefore, maybe Fuse was around 15 when he was writing this novel? If so, this is excusable.

Being a common reader with no writing background whatsoever, I am very unhappy to see how this novel could've been improved in almost every sentence.

Disclamer: I understand that I am not the one who got millions of sales of one's novel and, from some point of view, have no right to criticize the author. I humbly apologize before those who have this mindset. 

Haha, it's still an exploitative system. There is a reason Japan has the kareshi problem but hey, we can agree to disagree.
I think you underestimate the clout of companies like Kadokawa 

True but at least Chinese authors get paid unlike the ones on Syosetu.
It's pretty much slavery if you ask me.
I prefer the munpia model though where you are paid based on the chapters you sell, so you can't just pad chapters. Sadly, like in Korean dramas, characters are changed to cater to readers at the expense of the story 

I know exactly what you mean, slime tensei is not nearly the worst out there though, try to read Real Cheat Online more than half the time is is literally impossible to tell what is going because of vague nonexistent narrative, and heavy walls of unbroken dialog. 

What I've found that the problems you see in this book are fairly common in Japanese web novels.  The problem with them is that they tend to be written by complete amateurs with no experience in creative writing.  All they can do is draw on the material they read, and a lot of the material they read is just like this.  

The way I see it is that a lot of this kind of writing would be okay in a manga where you have other visual detail to add to the story, but it sucks when all you have to work with is the written word.  For examples, a lot of the expository material is taken care of by just seeing what the action looks like, and the first person problem is a non issue because the POV is handled by not being in your face.  The fact that many protagonists are purposefully designed to be nonentities doesn't help things.

There are better Japanese web novels out there.  Usually this happens when the writer can bring something extra to the story.  But as this tends to require either real life experiences or that the writer has interests outside of otaku material, it's not all that common to encounter.  I've found that Chinese web novelists tend to be much better when it comes to the mechanics of writing, but bear in mind that I don't read xianxia novels and these tend to draw the biggest complaints. 

That’s not slavery, that’s just desperation for attention and validation like wattpad. Except for a few who just want to experiment. Anyone of them could just put it up on Amazon instead if they wanted to. 

What irked me was from watching the anime.

All that, "Oh shit, he's attacking me. I should try to keep talking instead of beating him with my power". Never really paid attention to how much he talks while reading the novel.

Mind you, I mostly read CN novels. 

That's a basic "bait"-style to get the readers to want more, or to tease the readers. The Author can later explained them in future chapters, or in spin-off, or in some different series related to the original novel, or in some cases even forget about them.
And who know, did Fuse ever introduce himself?
If you are frustrated by that kind of writing, you aren't even a "common reader", but a "newbie reader", since it's one of the Most Common writing, and mostly used for Foreshadowing or stuffs.