The first detail: Eevee's fairy evolution method "Sylveon"!
In order to get Eevee's fairy evolution "Sylveon", You have to interact, pet, play, and feed your Eevee until it has 5/5 hearts, in the new feature introduced in Pokemon X&Y the Pokemon-Amie.
The second detail: The gym leaders of the kalos region.
1. The 1st gym leader is "Viola", she utilises bug type Pokemon, her powerhouse Pokemon is Vivillon. Upon her defeat she gives you the bug badge, and a TM called "Infestation" apparently it's a new bug type move.
2. The 2nd gym leader is "Grant", he utilises rock type Pokemon, his powerhouse Pokemon is Tyrunt. Upon his defeat he gives you the cliff badge, and a TM called "Rock tomb".
3. The 3rd gym leader is "Korrina", she utilises fighting type Pokemon, her powerhouse Pokemon is Hawlucha. Upon her defeat she gives you the rumble badge, and a TM called "Power-up punch" it's a new fighting type move.
Important note:
It was mentioned in a magazine leak of a Corocoro issue, that gym leader "Korrina" holds a secret for Pokemon Mega evolutions. At some point in the game, it is revealed that she has a Lucario that she's going to battle you with. Korrina's Lucario will become a Mega Lucario during the battle.
Another important observation:
At some point in the game you will meet a traveler who gives you a Lucario with its megastone the "Lucarionite".
4. The 4th gym leader is "Ramos", he utilises grass type Pokemon, his powerhouse Pokemon is Gogoat. Upon his defeat he gives you the plant badge, and a TM called "Grass knot".
5. The 5th gym leader is "Clemont", he utilises electric type Pokemon, his powerhouse Pokemon is Heliolisk. Upon his defeat he gives you the voltage badge, and a TM called "Thunderbolt".
6. The 6th gym leader is "Valerie", the first fairy type gym leader, her powerhouse Pokemon is Sylveon. Upon her defeat she gives you the fairy badge, and a TM called "Dazzling gleam" it's a fairy type move.
7. the 7th gym leader is "Olympia", she utilises psychic type Pokemon, her powerhouse Pokemon is Meowstic. Upon her defeat she gives you the psychic badge, and a TM called "Calm mind".
8. The 8th and the last gym leader is "Wulfric", he utilises ice type Pokemon, his powerhouse Pokemon is Avalugg. Upon his defeat he gives you the iceberg badge, and a TM called "Ice beam".
The third detail: The Elite4, and the Champion of the Kalos region.
1. The 1st Elite 4 member is "Wikstorm", he looks like a knight. Wikstorm utilises steel type Pokemon. The first Elite 4 member to use steel type Pokemons. Wikstorm's team consists of steel type pokemons such as Klefki, Probopass, Aegislash, Scizor.
2. The 2nd Elite 4 member is "Malva", she utilises fire type Pokemon. Technically she is the second Elite 4 to utilise fire type pokemon after "Flint", the Elite 4 member of the Sinnoh league. Malva's team consists of fire type Pokemons such as Female Pyroar, Talonflame, Torkoal, Chandelure. For some odd reason, "Malva" reminds me of "Lorelei" the Elite 4 member of the kanto league.
3. The 3rd Elite 4 member is "Drasna", she utilises dragon type Pokemon. Come to think of it, practically she's the third Elite 4 member to use dragon type Pokemon. The first one was "Lance" from the Kanto league, and the second was "Drake" from the Hoenn league. Drasna's team consists of dragon type Pokemons such as Drasna - Dragon-type - Dragalge, Altaria, Noivern, Druddigon.
4. The last Elite 4 member is "Siebold", he utilises water type pokemon. Siebold's team consists of water type Pokemons such as Clawitzer, Starmie, Gyarados, Barbaracle.
5. Finally! The Champion of the Kalos region "Diantha"!
Diantha's team is Hawlucha, Aurorus, Tyrantrum, Goodra, Gourgeist, Mega Gardevoir.
A screenshot of Diantha's Mega Gardevoir. A Mega evolution confirmed for Gardevoir.
The fourth detail: Aggron & Tyranitar Mega evolutions confirmed!
As you can see in this following screenshots, the left side is the Pokemon Mega Aggron, and on the right side is Mega Tyranitar. Both of the Pokemons are magenta shaded, which is why obviously you can't see their forms very clearly. It appears that 2 of these Pokemons are part of EV training.
5. the fifth detail: A new halloween ghost type move!
There's a new ghost type move called "Trick-Or-Treat" , it adds ghost type to the target. Making the target weak to ghost type attacks. A ghost/grass type Pokemon called "Pumpkaboo" learns this move.
The sixth detail: Team Flare's boss Lysandre (Contains major spoiler!)
The plot of Team Flare revolves around their research of resurrecting an ultimate weapon, created by the ancient king of the Kalos region.
Here are some screenshots of Team Flare's boss "Lysandre".
Important observation: Lysandre has a Mega Gyarados. A Mega evolution for Gyarados confirmed.
The seventh detail: Flabebe's evolutions!
Flabebe has 2 evolutions, all of its evolutions are pure fairy types. Here's the first evolution.
Floette
and, the final evolution of Floette is "Florges", it evolves into Florges if you use a shiny stone on Floette. Here's the final evolution.
Florges
The eighth, and the final detail: Honedge's evolutions!
It was revealed recently that Honedge has a new evolution "Doublade". Actually Doublade has a final evolution it is called "Aegislash". Doublade evolves into Aegislash if you use dusk stone on it. Here are some screenshots for Aegislash.
Apparently Aegislash has forms.