I think the most important settings are in regards simulation-distance, it is now lowered (edit: I thought that if sim-dist was set to 4, that meant 4 chunks would be "ticked", that isn't the case. So this probably won't be as much of a problem as I thought. The simulation distance is actually 9 chunks) and that an alternative redstone implementation is now used, this may or may not break some redstone.

The other two that might have an effect are: mobs will now despawn closer to players, and they are somewhat stupider (block changes won't make them update their path-finding). And some villager behaviors and sensors will now happen less frequently (this is the thing I think will have the least effect on anything)