Just more individual unique abilities, Not just pressure, maybe Lugia gets water stab and resistance, thunderous gets a chain lightning that reduces the power of electric attacks in double battles but all electric attacks hit multiple opponents, regirock heals when hit with a rock type attack because that's what it builds itself back together with, moltres is the Phoenix and all healing items affects are doubled (not moves like roost though) mewtwo x and y gain a small change of destroying the opponents held item on physical/special attacks accordingly.
