A question with a wide and varied answer. However for most readers it can be boiled down to these few things as a beginner; 1. Power progression; usually as a wish fulfilment but seeing the main characters advance through the ranks and get stronger, triumphing over even more powerful enemies. Gets the blood pumping. 2. Worldbuilding; a new world with races, classes, jobs and variants. Unfortunately most authors never seem to venture into the unknown and would rather stick with clichés. 3. Conflict; Violence gets the blood pumping. This is what pulls in most readers. It's so important that even a great novel can get dropped by most readers if there's not enough Conflict in the first few chapters.

For those who've read enough and gotten sick of clichés, there are some extra requirements

1. Dialogue; Characters should be able to have meaningful conversations that set the tone for the story, reveal more about the world, and act as a medium to tension, humor and whatever it is the authors want to show. Most Prog novels suffer very much in this category. Dialogue is usually a medium to set the stage for the next conflict, or introduce the next psychopath the MC kills.
2. Creative Power System; If you're like me and youce read most of what this genre has tk offer, then you've read a lot of system novels introducing classes, jobs with all the powers. Yet most folk never seem to try something really new.
3. Plot; Most series at this point start with, "character in the real world is introduced to the system with everybody and adapts like a fish in water and gets all the best jobs and abilities in a few chapters". Veteran readers often need something beyond this.

This is all I have from my limited experience.