PixlBit

So last week I wrote a bit of a thing on why Fairy type Pokemon and the changes Game Freak made to accomodate it are such a big deal in the world of Pokemon. If you haven't read it, check it out now so you're up to speed So now I'll get a little more specific with how specifically some will be helped and some will be hurt by this new addition. This is only speculation, but I'm going to give a sampling of Pokemon, some who will love X and Y and some who will hate it.

Helped

Gengar

I mentioned it before, but Gengar I think is going to really shine in X and Y. He's been great in every Pokemon generation so far (in fact him and Starmie are the only non-legendaries to be consistently great). Removing Steel's Ghost resistance means that he can spam Shadow Ball much easier. With this, only Dark and Normal Pokemon will give him trouble. 

The reason this is so important is because of Focus Blast. Focus Blast is a fighting move that has perfect coverage with Ghost typing. It also has terrible accuracy, making it really unreliable. Without a Steel resistance in the way, Gengar can opt for Sludge Bomb (Powerful Poison move) to hit Darks and Normals on the switch in. This can also be great coverage for Fairies, giving him great synergy with Dragons, Fighting, and Dark types. He won't be able to hit Tyranitar hard without Focus Blast, but between all of his support moves, he'll be able to annoy the hell out of Tyranitar and many others.

Scizor

While Scizor will not be a fan of yet another type resisting Bugs and the potential to switch in Fire types easier, he is going to love Fairies. His Bullet Punch (A Steel type move that always goes first and thanks to Scizor's ability, deals damage at over twice its normal punch) is already legendary in Pokemon and X and Y will offer a plethora of new targets weak to it. Add that most Fairies revealed so far either have a low Defense or look fast and frail and Scizor will be getting KOs left and right.

But he's going to hate not resisting Dark and Ghost after. Especially since it means he's no longer the perfect Tyranitar counter.

Azumarril

Azumarril already has one of the best abilities in the game: Pure Power. This doubles its attacks at astronomical levels. Yet there are so many good Water types it's had a hard time finding its footing. With Marril having an extra Fairy type though, it's safe to assume it will be passed onto Azumarril. This may be just the edge the little guy needs to be a terror.

With this, it may be the perfect bulky water to stop Dragon types. Meanwhile his Fairy typing will buy him resistances to Fighting and Dark as well in exchange for losing his Steel resistance and gaining a Poison weakness and he looks to be a very good anti-metagame Pokemon. Add a Water priority attack to bypass his poor speed I can see him being a perfect counter for Tyranitar, Conkeldurr, Keldeo, Hydregion, Haxorus, Heatran, Blaziken and maybe Garchomp as well as being a really good check to Infernape, Dragonite, Salamence, and others. Most would kill to beat those guys one on one.

Dialga

There are many things about Dialga that will ensure its success, but the huge one is that it's the only Dragon who won't be weak to Fairy and can hit them hard back. That is invaluable.

Heatran

I get the feeling Heatran's going to play a key role in countering Fairy types. He'll be packing Flash Cannon as an alternate, but that will be a small price for being able to come in, shrug off anything Fairies throw at him, and hit them back hard. Every Dragon type will want a Heatran or Scizor for support. Every Fairy will try to have a way to take them out.

Bisharp

Steel no longer resisting Dark and Ghost leaves Bisharp at a unique position: He's now the only Steel type to resist those two still. With what I said about Gengar before in mind, that is huge. I can almost guarantee Ghost types will initially forgoe their hated Focus Blast and when they do, Bisharp as a Pokemon mostly forgotten before, will come in and they will have no answer for him. His speed and typing held him back in Black and White, but his typing may be what makes him shine in X and Y.

Charizard and Absol?

These two are lumped together and a question mark because of their Mega Evolutions. Mega Evolution importance in X and Y is still a huge question mark and depending on how convenient they are, could play a huge role in it. Both of these could be key players for different reasons. Charizard's new ability would make him summon sun switching in, giving a better option than Ninetails for the role. Absol, meanwhile, may get the stat boost he needs to abuse his attack. Meanwhile Tyranitar will not enjoy all these Steel types leaving a niche for something to hit Psychic and Ghosts hard without being weak to Steel. Cannot say for sure yet, but if Mega Evolutions end up being an important part, these two have the most to gain.

Hurt

Every Dragon Type

The whole reason Fairies were made: To fuck over these guys. Dragons are going to be taken down quite a few pegs. They'll still be good, but countering them will be so much easier.

Hydregion

A special mention goes out to this Dragon. A quadruple weakness to Fairy and every move currently used by it is either resisted by Faires or they're immune to it. Yeah, this thing is toast unless you can take out every Fairy first.

Tyranitar

Tyranitar has so many weaknesses that adding another puts it in the point where almost everything will have something that hits it hard. It can hit Steels for neutral damage at least now, but Steel attacks will be more prevalent than ever and hit him hard. It'll still have its use, but not as much now.

Metagross/Brongzong/Jirachi

These three are lumped together for being fucked for the same reason. Their once amazing Psychic/Steel typing, one of the best in the game, will now have two weaknesses added. They're going to take a serious hit. Jirachi may survive if it becomes Steel/Fairy (which I could see happening).

Mamoswine

Mamoswine's niche before was being a fantastic answer to every Dragon while also having an Earthquake that really, really hurt. Well, now we have a better answer, more Steel types to hit it hard, and an easier time for Fire types. Sure, both are weak to Earthquake, but Mamoswine's second Ice typing may have just outlived its usefulness.