I played table top AD&D first edition and the Basic-Expert DND long ago. The thing that confused me most in the beginning of BG3 were the spell properties, like concentration and duration. Also the distinction between turns and rounds I knew from the old school DND is no longer strictly used here, so that was confusing also.
For instance we have the mage armor spell and the cleric shield of faith spell which are very similar, but the cleric spell is concentration based and the mage spell isn't, so one kept going on and the other mysteriously disappeared.
After 900+ hrs of playtime, (having bought EA in november 2020) I 'm still struggling with the effects of magic items. Some are so totally designed to work- with certain class/spells combinations, I have trouble to really evaluate what the total combined effect can be.