Rules changes in brief:
Many of the changes are going back to SST's rules than brand new changes - Fire zones have to be centred on a model now, you can ready as a reaction and "save" readies between turns. Special moves are back in (many units have a special move type (wheels, tracks, jump, or hover) that you have to ready to use, but can shoot during the move), cover is the main new thing - there's three levels of cover - obscured, light and heavy, with an increasing bonus to your armour save and kill score depending on how heavy it is.
The other main change is how the army lists work. Instead of picking an infantry or armour platoon and then having a set number of assets per squad/armour choice, now an infantry platoon can take 0-3 batallion (AKA support), Armour or Air assets. Transport assets are gone, replaces by the option for a squad to take an APC.
Armour platoons are more on the squadron/company level, with up to 9 armour assets allowed per "platoon" with 0-3 squad, batallion or air force assets.
Some forces vary from this template, and the MEA are wildly different.
