I haven't read the novel and have no plan to, so you won't see any details here other than that related to the manga itself.
This manga is at it's core wish-fulfillment, in that like previous mentions is what a rpg gamer wants out of his game. Slowly leveling up, gaining new abilities, fighting bigger and bigger monsters, getting the girls, interacting with npc's...you get the list, it's all here.
There seems to be a lot of confusion to the fame of this manga, and honestly its quite simple. Many people picture themselves as the protagonist and thus going through a journey in this heavily narrated tale is like doing it yourself. The story obviously won't go down as the next Romeo and Juliet, but it's simple and detailed enough to fully immerse yourself in the world.
This honestly breaks the mold in manga and novels (fantasy ones), with a gary sue like character. It's almost impossible to find a character that wins a battle because he just that good, it's always an ass pull, lame as training for a specific attack that he'll do that one time or sometimes when the author can't think of anything else: plot armour saves them.
One major problem though with these kind of characters (gary sue), is boredom - predictability. So far, it's just been setting the scene for the rise of the goblins, so we haven't encountered the problem yet, but when we do get to it, I'd like to see how it's dealt with.
I don't like calling people out, but the review by Immortal bear is filled with so much malice that I can't over look it.
- The goblins are a weak race and any resemblance of strength comes from Rou (from formations to training)
- Golbins don't gain powers by eating things, that's what makes Rou special
- We have yet to see what Rou's backstory has to do with his current life, so we can't say it has nothing to do with the story
- Rou is not a twisted character, he doesn't eat the flesh because he likes it, he does it out of necessity (he needs their powers). The parts where he ate the women that golbins rape is not shown, further showing the obvious disdain from the act the author and in extension Rou has for it
- There have been no sex scenes of yet
- I agree that so far some of the side characters are kind of 1-dimensional, but for every character you show that is one dimensional here I can show you 20 from a plethora of other mangas
- It's kind of obvious that the girls would fall for him, he's just that strong, handsome and a great guy. I've seen shallower relationships form in real life, my mate and his perpetual need to swat away the amount of girls that are in love with him is testament to it
TL;DR if you don't understand why this appeals to such a large audience and rakes them in, then the human psyche isn't something you understand.