I haven't done one of these before, and I have someone beside me talking so lets give it a shot. (I typed article, smooth).
With the release of X and Y, a lot of pre-testing (before set release), The format now seems at a very autowin/autoloss ratio. There is other things such as skill, how strong you're decklist is, etc. After a lot of play testing in the format with 2 decks that not only 1. won a lot of tournaments and 2. lock the crap out of 50% of tier decks and about 90% of the under tier2 decks. With that being said these decks also have auto losses and auto wins to themselves and the rest of the format, so right now this is how it looks like..
Regardless of the loop, there are other things in the format that make up the autowin/autoloss.
Virizion, with ho-oh energy accel and type advantage becomes very strong, muscle band makes up for a lot more dmg options in the deck, ho-oh then added with a few cards for specific type advantage depending on whats played gain a lot of powerful leads with the deck w/o the use of having to worry about using the EXs to win with.
Blastoise. seems to not be as good as it used to be, with it's matchup to virizion losses on type advantage and reliable catcher effect. Losses to rayboar on a few things but majorly the use of dragon basics. Although this deck is very strong and can autowin a lot of decks in the format I find on pure luck based draws which can be increased by the amount of cards needed to pull of very specific combos. This deck isn't terrible, it can still take a win here or there, time will just have to tell.
Plasma, is still considered a strong deck in the format but takes heavy hits from the rest, even though it has made some major showings at regionals, it had problems finishing. Walking into x and y it seems like a lot of attention has sort of left the deck which might give it an edge on winning in the coming months. This deck's strong matchups are against decks like boar, stoise due to frozen city, thundurus has the potential to ohko yveltal due to weakness, with 4 deoxys, muscle band and a lugia take 3 prizes from darkrai on a good day, giving a lot of decks the run for they're money.
Darkrai. Going into X and Y and gaining yveltal, darkrai is still as strong now as ever before. Testing has shown that it makes darkrai a much more diverse opponent even though the bird takes an autoloss to thundy and raichu, with the stadium seems to be the least of it's problems. Regardless of it's autoloss functions it gains a lot of momentum against decks like virizion, plasma, dragonite, etc decks that utilize a lot of energy to attack with. either paring this with garbo or dusknoir can make this deck one of the most revered in the format.
Trevenant. One of the strongest lock/disruption decks in the format. With decks like redcard/honchkrow/hooligans, darkrai/malamar/redcard, etc doesn't hold much of an autowin to the rest of the format as they think, with cards like delphox or electrode they really don't gain much if they lose 3-4 cards and feel much more inclined to setup faster putting they're opponent in bad situations. Raichu/dugtrio, etc, really puts alot of these shenanigans decks into perspective as the first person to setup wins, however you still lose regardless to a strong opponent with a strong deck, which leaves you at the folly of the 2-2-1 record place at the middle of the tournament. Where as trevenant is much safer at locking the opponents active and with the loss of a heavy evo line in which gothitelle took up now allows a lot more for stuff like preferred or effective techlines to accel at winning such as muscle band, virbank (to now guarantee ohkos against keldeos), crushing hammers to shutdown silly techs like the 1 of virizion in you're opponents deck, you could always just keep on catchering stuff and locking it into place, dusknoir to keep the lock going. All in all the deck has a ton of options at making it work in the format.
ultimately at the end knowing what wins to what and lose's to what can help you choose how and what to make the best options against the format you play in, or make the most amount of leeway. But even if you're deck favors a strong matchup against specific decks, testing helps in insure that is what happens, also playing you're deck for it's weaknesses helps overall in making certain it doesn't always take autolosses.
Decks I left out such as fairy didn't really show a strong competitive deck that answers a lot of questions to the rest of the format, and or give the deck a lot of pressure, and because of it cobalion will be a complacent thing in a lot of decks, hell even 6 corners might come back, we'll just have to wait and see.