The most interesting point about the new rules FAQ/Errata is that GW used the opportunity to change actual rules (not just clarifications) AND most importantly reaffirmed some rules that were already in the book. GW took this rules update as an opportunity to stand by 8th edition.

The first rules change they made (not really an errata or FAQ, but an actual complete change) is that now monstrous cavalry get the highest toughness of the 2 components instead of the riders toughness. This is a huge change for Empire Demigryphs and potentially any monstrous cav given in the future to elves.

The other important ruling they reaffirmed was that ward saves DO NOT APPLY from a rider to his monster mount or vice versa. While this rule was already in place, GW putting it in the FAQ is a clear indication that they like this rule and are going to stick with it (obviously, they don't believe cannons/war machines are too powerful).

Many other updates in this one (entombed beneath sands not required, Hurricanums don't stack, no overrun from instability, banner of rage gives increased attack to mount too, etc).