One of the coolest additions to Omega Ruby and Alpha Sapphire is a certain Pokémon that can mega evolve without having to hold a mega stone. It's also a tweak that kind of breaks said Pokémon.
Meet Rayquaza, a flying/dragon type Pokémon. If you've followed any of Omega Ruby and Alpha Sapphire's marketing, you know who this is—he's the third big legendary in the games, next to Kyogre and Groudon. Kyogre commands the sea, and can make it rain. Groudon can raise land, and can turn up the heat. Rayquaza might not be on the covers of ORAS, but he reigns above both of these legendaries. It's all thanks to an ability called "Air Lock," which can negate weather effects that Pokémon like Kyogre and Groudon bring into battle.
Rayquaza has always been a strong Pokémon—most legendaries are. An article written in 2012 on Smogon, a hardcore Pokémon community, says that Rayquaza "can blow past the competition," and that Rayquaza is "one of the single most dominating offensive forces to watch out for." With a high base attack and special attack stat of 150, this is a legendary Pokémon that has always been able to pack a punch. For comparison's sake, the Pokémon with the highest possible base attack stat as of this writing is Deoxys, which has a whopping 180 base attack. Deoxys is also a legendary, though. A more typical, everyday Pokémon like Charizard has a base attack of 84. Rayquaza's attack might not be the highest, but a combination of good stats and a good movepool make him a fearsome foe anyway.
That was true back when Rayquaza was originally introduced into Pokémon in 2003. Now, with the release of Omega Ruby and Alpha Sapphire, the situation has only gotten more intense. The extra "Delta Episode," available once the main ORAS storyline is beaten, gives players the chance to capture a special Rayquaza with a move called "Dragon Ascent." Unlike the Rayquaza in the original games, this new Rayquaza can mega evolve thanks to that move. Mega Evolution, as you might know, is a process that can beef-up and transform Pokémon temporarily—but only if the Pokémon is carrying the appropriate mega stone. This is a mechanic that ensures that mega evolved Pokémon can't get too much of a boost, since carrying a mega stone means the Pokémon can't carry a different item, which could boost its stats even more. This new Rayquaza is special in that ORAS allows it to mega evolve without carrying a mega stone. The lore justifies this by saying that he has a "unique biology." In practice, what this means is that not only can this version of Rayquaza grab a boost from mega evolving, he can also equip an additional stat-boosting item on top of that. Did I mention that Rayquaza's new ability, Delta Stream, makes any moves that are typically powerful against flying-type only deal 1x damage, instead of being super-effective?
We can make an estimation of a Pokémon's overall strength by adding all of its stats, something which is denoted by the abbreviation "BST." To put this into context, normal Rayquaza has his HP at 105, an attack of 150, defense of 90, special attack of 150, special defense of 90, and a speed of 95. Added altogether, he has a BST of about 700. That's typical of Pokémon of his calibre.
Depending on its stats, as well as the state of the competitive meta game, a Pokémon might find itself in different "tiers" determined by competitive communities such as Smogon. Think of them like weight classes in boxing, only for Pokémon's competitive battles. So far, tiers include "OverUsed," "UnderUsed," "RarelyUsed," and "NeverUsed." As the names might suggest, the tiers are based on frequency of usage—save for one. There is also a tier called "Ubers," which is where a lot of legendary Pokémon get put, provided they're powerful enough. Each tier has special clauses which inform how players battle.
Last weekend the competitive community announced that they were making changes to the tiers, and it's all thanks to Mega Rayquaza. It all comes down to Mega Rayquaza's stats:
In general, the Ubers Pokemon hover around ~700 BST; anything higher has either a wasted ability slot (Multitype) or a restricted item slot (Orbs, Plates, Mega Stones). What separates Mega Rayquaza from the rest is the fact that it is the first unrestricted ~800 BST pokemon. As a result, it is able to easily overpower most Pokemon in Ubers.
So in order to account for this massive stat boost, Smogon has incorporated an entirely new tier: "anything goes." There are no special stipulations in this tier—no banned Pokémon, or movesets, or strategies. Like the name says: anything goes, including calling on the overpowered Rayquaza. You can read more about this decision here.
To the average player who isn't into competitive Pokémon, changes at Smogon such as these won't matter. It's not like you have to worry about things like stats in the single-player of Pokémon. These tiers and rulesets are not the sort of things you'd need to worry about in official Nintendo-sponsored competitions either—those have their own rules and regulations. But this change in the competitive community is a notable one, as I've seen a lot of complaints around mega evolutions, and how they're given to Pokémon that are already cool and powerful. Mega Rayquaza is proof of that.
But, more importantly, if you're playing Omega Ruby and Alpha Sapphire, chances are good you're going to capture Rayquaza. And if you've been happily using Mega Rayquaza this entire time, now you know why he seems to sweep through every poor Pokémon that gets in his way. No wonder the ancient peoples of Hoenn prayed to Rayquaza to save them from disaster. He's overpowered!