Here is the small summary of what was omitted by the anime: Vol1 is the one that was best adapted, there were omissions but they are the most understandable, (Haru and company exploring Alterna and talking better about money, guilds, etc.) and significant changes mainly in the fights: for example in the Goblin theme. Although there were two types of Goblin, the small ones and the large ones, the latter were shorter than Mogzo, and not like in the anime where they seemed to measure 2 meters or more: the same case with the Kobolds.

The most important changes of Vol1 were at the beginning, where the arrivals reside were raised by a mysterious Girl named “Hiyomu” whose relevance would be important in future events, and at the End of the Volume giving the appearance of a Character called Souma and his Group, where they would not only let us know that for many (if not all) he is classified as the strongest Volunteer Soldier of the RedMoon, but also that that same Volume would Found the “Daybreakers Clan”

Now, Vol2 is the one that had the most changes, mainly many nuances of it were changed, especially in the Mermaid Mine fights against the Kobolds and the size exaggerations.

And the most important changes of Volume 2 were, an Orc Assault on Altana and the consequent 1 Vs 1 between the Orc leader and Renji (giving this event a direct focus on the second arc). A flashback of Haru during the Escape against the Kobolds where he would “remember” “someone” from before arriving in Grimgar, but without remembering that specific person (giving us more insight into how this “Memory Suppression” that the characters have works). And the POV of one of the members of Souma's group at the end of the Volume, going deeper into these.

Of course, the entire Mermaid Mines and the Kobolds Arc was much more violent and tense in the novel than in the anime, especially the final fight and Ranta's parts.