Things I liked:
Not tutorial heavy. (Why, oh why, do the tutorials always go on for sooooo long? If someone's reading LitRPG/dungeon core, it's a safe assumption they already know how these games work.) Especially like how Erin the fairy sucks at her job, and gets defensive about it, which is both funny, and a clever way to stop the first third from being an infodump on the mechanics of how the core accesses stats screens, constructs mobs, etc.
Skeleton fight club! The humour in general. Puns, jokes, pop-culture references. Enough to be fun, but not so much it's tiresome.
Genuinely likable characters.
Well written, well edited, story has narrative tension, conflicts and obstacles to overcome, character growth, and delivered a solid and satisfying conclusion. (This seems pretty consistent across books published by Portal.)
Things I didn't like: nothing really. This book was pretty solid fun.
Four stars: really enjoyed, looking forward to the next one; highly recommended, especially for people looking for an enjoyable entry into gamelit/dungeon core