How can such a "basic" show could be so popular? Looking at the rates of similar series, how can this stand above them all? The answer is easy: Because it does nothing wrong.
The idea is simple: Kaede, a normal girl who hadn't played RPG in her life, begins playing a virtual game due a recommendation of her best friend. As she didn't know how to play correctly, she just became a shielder and maxed out her defense, without improving anything else. Even if this sounds pretty difficult, she just does whatever she wants and takes advantange of that.
FINALLY we have a fun and interesting anime focused on the events, worldbuilding, character development and how enjoyable could a MMORPG be. You don't have a complex history, you don't have awful fanservice scenes, you don't have boring drama... You just have a good show about playing games!
If you are usually disappointed with this kind of works, I really recommend you to give it a chance!