And like Primal Kyogre and Groudon, it has a new ability, Delta Stream, which is an improved Air Lock in that it actually cancels weather instead of suspending its effects, AND weather cannot be induced whenever Mega Rayquaza is on the field. It even works on Desolate Land and Primordial Sea, making it what Air Lock always should've been in the first place. I think we're all relieved that Gamefreak didn't botch this. Additionally, Delta Stream also has an extra effect: it cancels Flying-types' weaknesses. However, the wording is rather ambiguous. Does it straight-up halve the damage of types that Flying is weak to (meaning Mega Rayquaza would resist Electric, for example)? Does it halve the damage of types Flying is weak to, but only if the other type doesn't cancel the weakness (so Electric would hit for neutral damage, but so would Rock)? Or does it cancel out any weaknesses Flying-types have, including those from their other type (giving Mega Rayquaza no weakness)? Either way, this gives Rayquaza an ENORMOUS buff that will no doubt be able to allow it to come back from the obscure depths of the uber tier. And it gets a brand new move as well, Dragon Ascent, which is apparently the most powerful Flying move in the game. In layman's terms: expect a drawback of some sort. Hopefully it's one that isn't too crippling, because God knows Rayquaza needs a good Flying move.
Edit: Dragon Ascent is confirmed to be a Close Combat clone, although its actual power is up in the air. Probably the best possible choice for a drawback, because Rayquaza's really frail anyway.