The Abilities full descriptions have been announced for both of the legendaries:
http://www.pokemon-sunmoon.com/en-us/pokemon/solgaleo/
http://www.pokemon-sunmoon.com/en-us/pokemon/lunala/
Solgaleo’s Ability is Full Metal Body, a new Ability that no previous Pokémon has had. With the Full Metal Body Ability, a Pokémon’s stats will not be lowered by the effects of an opponent’s moves or Ability. Notable: Its signature move is Sunsteel Strike, an attack that charges at an opponent with the force of a meteor, disregarding the target’s Ability.
Lunala’s Ability is Shadow Shield, a new Ability that no previous Pokémon has had. With the Shadow Shield Ability, a Pokémon will take less damage from an attack that lands when the Pokémon has full HP. Lunala’s signature Moongeist Beam attack releases an ominous beam of light that disregards the target’s Ability.
Just quoted from the aforementioned links to the us site
short version: Full metal body is a better clear body, immune to sticky web, intimidate, seed flare drops, etc. Shadow shield is multiscale, but it may prevent reduce damage by 75% instead of 50% like multiscale, but I doubt it.
I also found out what Shadow Shield does: it halves damage taken while at full health, like Multiscale.
This could actually be potentially useful. What it means is that anything that isn't a Ghost or Dark attack effectively is resisted by a fresh Lunala, and since it'll likely get Moonlight and/or Roost, it could make it surprisingly tanky. If it had some key advantages over the other two Multiscale users (Lugia and Dragonite), namely better offensive presence than Lugia and better speed than Dragonite, it could be quite formidable.
Also I just realized Lunala kinda looks like that Psychic leader in Kalos, Olympia.
Kiiiiind of a bit late there aha :P
