First some items I need to adress. I'm Dutch to expect some spelling errors, if they are to annoying just message me and I'll fix it.
My opinion may vary from others and this is fine. People are different so there is a good chance that I will not like what you love. Deal with it. I'm not telling you that you should hate it too, I'm telling you why I didn't like it.
I'll try to keep swearing to a minimum but fail utterly on occasion. In general it is the more the anime pisses me off the more I swear so watch out for the low scored ones.
Art: It's fine. It's so very hard these days to screw this part up with the use of CGI in sp many shows. Not outstanding but certainly not bad either. It does what it needs to do.
Sound: I very rarely pay any attention to the music. It should be there but not overshadow what is on screen and in that regard it succeeded. Voice acting is fine.
Characters: The main character is Momonga. As the guild leader of Ainz Ooal Gown he decides to stay in the game untill the servers shut down. He clearly loved the game and the guild that he and his friends build.
Yggdrasil is a massive online rpg that had millions of players playing it. Now, 12 years later, it is shut down.
Ainz Ooal Gown was a small guild with only 41 members but they were considered to be the strongest in the game. The members were known for playing monster characters, Momonga being a Lich.
Story. After 12 years the DMMORPG Yggdrasil is shutting down as most of the players have moved on. Momonga is one of the few members of the guild left and waits for the end of the game. When the clocks strikes zero he is therefore very surprised that the game is still there. He quickly realizes that something is wrong as he can no longer log out or send any messages to other players. The NPC's in the game have also changed and appear to be self aware now. Stuck in a world that is both the same and different, Momonga, decides to take things slow and figure out what happened. His hope is to find others like him and find a way home but first he must survive this new world he finds himself in.
Rant. This genre of someone stuck in a game unable to get out is not new. We have Sword art Online, Log Horizon and more than a few others who do the same. I personally like the genre and there is a whole slew of literature that popped up when the genre got popular.
If this one has a twist it is that Momonga is stuck in an unhuman body as a lich. That means that he is essentially an undead and incapable of feeling emotions. During the series the world repeatedly intervenes when his emotions run to high and they get cancelled out. This plays mostly out when it is convenient for the plot and otherwise doesn't play that big of a role.
Another issue is that Momonga was the maximum possible level in the game and seriously overpowers any other NPC in the game. He does learn that there are items left that can hurt him but his clan has a storeroom filled with such items so it barely plays a part in the anime.
The anime desperately tries to set up some threats to Ainz Ooal Gown but reality is that nothing comes close to the power that the clan wields.
There are 3 seasons so far that I have seen and the first one is the best of them all. It sets out the world of Yggdrasil and the people within it. Together with Momonga you try to learn what is going on and how the world around him has changed.
I once saw one of those top 10 lists on YouTube where they showed main characters that turned out to be evil instead. Momonga is on there for something he did in the third season. I disagree. Momonga became evil in season 2. You could argue that he never was good to begin with but the point of no return was season 2.
Also the summary on the DVD box of season 2 is laughably inaccurate. The lizzard people didn't rebel against Ainz Ooal Gown as he wasn't their overlord to begin with. They were living peacefully in the swamp and Ainz Ooal Gown simply wandered in so that he could enslave them for his indead army.
As a consequence I disliked season 2 a lot more as it wasn't something that I would have done. Still there was also a lot of good here and I liked many of the side plots and characters. Sadly at this point Momonga is possibly the least interesting character in the anime.
Season 3 is all about Ainz Ooal Gown and the kingdoms they find themselves wedged in between. He sets out to make himself known to the world in general and set himself up as a king of his own country. Same issue as I had with season 2 is that Mononga is the least interesting character.