As mentioned above, Fairy has had it's type chart revealed as well as Steel losing two solid resistances to Dark and Ghost. To start off on Fairy, I believe that its weaknesses will actually help bring two less seen offensive types to greater prominence, as Poison and Steel were hardly ever used as coverage before, and only used as STAB moves when absolutely necessary. Poison I think gains the greater boost from Fairy being weak to it, as Pokemon like Gengar and Venusaur suddenly have an alternate STAB that is quite useful, as Fairies will be very prominent in the new metagame (at least at first if not beyond that as everyone will be trying them out for the novelty of using one). Steel on the other hand has not gained too much from being super effective, it just gives Scizor yet another thing to be good at, as well as Scarf Jirachi and such. I do not see too much use of Steel coverage moves used because 1) very few Pokemon actually have Steel coverage that is actually any good and 2) there are so few Steel moves to choose from (unless they remedy that in XY by giving us a nice variety). Being super effective to Fairy though is completely overshadowed by Steel losing its resistances to Dark and Ghost, effectively crippling many solid cores in addition to likely affecting the usage of several Steels.
To start off the list of what is going to become more prevalent in XY competitive, one of the more worrying things will be Gengar. Gengar now has nearly perfect neutral coverage with its two STABs (so does Bisharp but not sure how much he'll be blasting through) and has the toys to abuse it with. Being resistant to the new Fairy type is a great asset as well, and the ability to hit it super effectively with Poison STAB will make Gengar formidable. It also has high Speed and Special Attack, being able to now 2HKO Specially Defensive Jirachi with Shadow Ball, which considering it has a chance of 3HKOing at best currently (and this is with Hidden Power Fire and not even completely guaranteed). This is enormous power to hit Steel types that Gengar did not have before, and may also mean that Gengar will no longer need to run the burdensome Focus Blast. Moving on to our next major threat is Tyranitar. Now that Steels have lost their Dark resistance, this gives Tyranitar the ability to Pursuit many Steels with impunity, with Choice Band Tyranitar being able to OHKO Jirachi with Pursuit if it attempts to switch, and still do enormous damage even if it doesn't (yes, I know Jirachi is getting the brunt of the abuse here, but bear me out lol). Several other Steels will also have some worries about Pursuit/Crunch, including our friend Honedge who used to have the great typing of Ghost/Steel, the typing not being so great anymore. I also see Ghost and Dark coverage becoming a little more common to 1) hit Steels and 2) hit Ghosts which will be a bit more threatening this upcoming generation.
To move on to the Pokemon that took the hardest hits from this announcement, we'll start with the Psychic/Steels. Jirachi, Metagross, and Bronzong now all have two more additional weaknesses, both being easily exploitable and extremely damaging. Jirachi is going to take the hardest blow, as it was an extremely commonly seen Pokemon in OU this generation, whereas next generation it will struggle a little. It does have the advantage of being able to hit Fairy types hard, but I'm not sure this will negate the enormous weaknesses it now has. Metagross, who has been in danger of falling from OU for months, will likely be finally pushed over the edge into either BL or UU, depending on what sort of Pokemon also end up in these tiers. Bronzong will also take quite the blow, not being the great defensive Pokemon with very few weaknesses anymore, and instead having very common ones. The next types on my list should be relatively obvious, and those are the Dragons. The Dragon types have received ridiculous nerfs with the advent of XY, not only being weak to Fairy types, but also having Fairy be immune to Dragon. This quickly will increase the riskiness of using anything resembling DragMag (especially with Steel having been nerfed as well), and Outrage spam will be unwise as any Fairy can switch in and ruin your day. Hydreigon probably takes the hardest blow, being 4x weak to Fairy and having both its STABs be resisted in the case of Dark, and completely blocked out in Dragon's corner. Other Dragons that commonly run a Choice item like Garchomp and Latios will also have to be careful about what move they use. Some honorable mention from the rest of the typing info are that of Fighting types (weak to Fairy, Fairy resists), Bug (Fairy resists) and Fire (resists Fairy). Each of these three types could hold more Pokemon that rise or fall, especially depending on whether any more older Pokemon are reclassified with Fairy type.
Moving on to the newer Mega Evolutions, I will just briefly cover Mega Mewtwo X as we have very little information on what it actually does. We do know it's Psychic/Fighting, which could be very useful depending on the stat spread it receives, as STAB Aura Spheres would become very useful coverage in Ubers. Unfortunately, we don't know too much beyond that besides the fact that it gets Steadfast for an ability, but to be honest there really isn't anything out there that will flinch Mewtwo anyhow. We also don't know what stats are getting boosted, so we have no idea which way to speculate. Mega Garchomp is the real beauty that we want to discuss. Receiving Sand Force is extremely useful for Garchomp, as it means its Earthquake becomes immensely more powerful. In fact, at +2, Sand Force Garchomp with its current stats can basically 2HKO the whole of OU, mostly OHKOs. And this isn't even factoring in the likely stat boosts to Attack or Speed (or both) that Garchomp will receive in its Mega form. Unfortunately, Mega Garchomp has been born into the era of Fairies, meaning hitting things with Dragon attacks won't work nearly as well as it used to, but the huge boost to Earthquake will likely make up for it.
I would discuss the starter evolutions as well as the new Fossil Pokemon and just all the other new Pokemon, but we really do not have much to go on with them, no stats, no movepools, not even complete evolution lines. Speculation based solely off the typing and ability just doesn't work, so I will forbear (wait, didn't I do this with Mewtwo :$). In my opinion, as Barbeller was saying, it looks as if Gamefreak is gearing us towards an even more offensive metagame than 5th generation. Yikes. To be honest, I really don't mind that much, but the death of Stall is pretty sad to see, unless Gamefreak releases the defensive behemoth known as Mega Skarmory (hey, we can always hope). As the last batch of Corocoro that releases before XY, the news is going to be slow so there really won't be too much to discuss after Pokemon Smash airs on Sunday meaning there are actually going to be quite a few surprises waiting for us in the games, unlike this generation where the whole Pokedex got leaked before the games. It will be very interesting to see how the Pokemon we already know about end up, as well as seeing all the new ones and how they'll fit in on a competitive level, but until then, this is basically all we have to work off of.
tl;dr: Steel got smashed, Fairy will be a mixed bag, Gengar will be god (Bisharp may end up a demigod), goodbye HaxRachi, Pursuit everywhere, GARCHOMP HAS SCYTHES, and let the slow news period begin.
Lol, I didn't realize how long this thing was until I posted it, maybe you should all just stick with the tl;dr XDXD