First of all, if you don't like "Big breasted anime girls or anime lolis" then you're probably in the wrong genre of entertaintment. Show me an anime that doesn't have one of those, maybe the 0.001%?. So I don't see how this is even relevant at all, it's like saying you don't like Pizza because it has cheese or 2/10 too much water.
The Anime itself it's really good and very similar to one of my fav animes of all time. The legend of the shield hero (Tate no Yuusha) it's basically the same thing, guy gets send to another world and he starts out as a worthless monster (In this case with a couple of really good abilities) and eventually grows up to become incredibly powerful and a saviour to those around him. If you liked shield hero then I reccomend you watch this.
Where this Anime is special is in it's positive attitude. One reason I got tired of Overlord is because of how arrogant and belligerent he becomes even though he was originally a human himself. The light novel is also a lot of murder and violence and altough that's cool depending on context (I love Berserk) I just don't see the point of an all powerful invincible creature toying with opponents that are weaker than himself. It just comes off as cowardly to me, (No offence to those who love Overlord) but it's really easy to be a bad ass when you live in a secure place and nobody can fight you back.
Reincarnated as a Slime is the complete opposite. He uses his powers to help others and it's always trying to do everything the best he can. It's just fun to watch and you want him to make the world around him a better place.
Is he a Mary Sue? No, I don't think so. A Mary Sue is a perfect being, anything she wants to do, she can do, perfectly. She doesn't earn anything she was just born that way and everyone else is inferior in comparison, she doesn't need anyone but everyone needs and loves her because she's the greatest ever.
Remuru, on the other hand, worked for his abilities. At first he couldn't even see or detect magic. The reason he's so powerful is because of Verudora. He relies on his allies all the time and it's not just out there looking out for himself (In this case Overlord is more of a contenter for Mary Sue than Remuru is). People also don't love Remuru for free, they love him because he earned it by helping them out and listening to their problems. Remuru grows and has a Hero's journey, not only that, but he also helps others grow with him when Mary Sue's don't. He respects other people's authorities, like the Dwarf king, and didn't try to fight back even when he got injustly thrown in jail. These are things a Mary Sue would never do.
In Conclusion:
This is a feel good, power fantasy slightly harem kind of Anime where everyone is just trying to get along and make something for themselves. It's like a coming of age type of story, a guy who is a worthless salary man, gets a second chance to be reborn and become someone powerful that can help those around him. Wishfullfillment? yes but who cares? Life sucks enough as it is, so light hearted stories like this are great once in a while.
This Anime is fun to watch and I like it almost as much as Shield hero. I will give it a 9/10 and reccomend everyone to at least give it a try and ignore all the dumb boob haters.
