Firstly, welcome to the server! I hope you enjoy your time here.
The biggest change is the fact that this version is only updated up to G13s2. As a result, you do not have the fast paced combat system of Official mabinogi, instead you have the loading time combat. We do have content that is implemented from future G's though like special upgrades, specific egos and certain dungeon drops that were available later than g13s2. However, there is no fighter, gunner, bard class (there are no plans to implement these as it is impossible) and there are no "talents", but rather the old destiny system after your first rebirth. We have 3 custom shadow missions that do not exist on Mabinogi official. You also have no "cash shop" for irl money, instead you have Red Fomarian Coins which drop from monsters, which allows you to buy pets/gachapon/dyes etc. There are no AOE pets, so no dragons, flame horses, etc. Fashion is being implemented into the game consistently and there is much more focus on crafting. Fashion that becomes obtainable usually does by being released as a pattern/manual instead of something you just get out of gacha. A major difference also is that the GMs listen to your suggestions and run content polls to see what the community does want to be included or changed in the game. You can often speak to them on the Unofficial Mabi-Pro discord server or ingame.
I may have missed some things out, but I hope this gives you a good idea of what the server has to offer and what the differences are