Weakness Policy, Held Item ,Raises Attack and Special Attack sharply if the holder is hit by a move it's weak to.
Assault Vest, Held Item, Raises Special Defence but prevents the use of Status moves.
Whipped Dream, Evolution item for Slurpuff
Sachet, Evolution item for Aromatisse
There's a lot and I don't feel like doing them now.
Weather-causing abilities (Drought etc) are no longer permanent - the weather fades after 5 turns.
Critical hits now only deal 1.5x damage.
MegaEvolution appears to take priority over attacks, not just your own. For example, the following order of events is possible:
Pokemon A MegaEvolves, Pokemon B uses regular priority attack, Pokemon A uses regular priority attack.
Only one Pokemon on a team can MegaEvolve per battle.
If a Mega Pokemon is switched out, it remains in Mega form.