As Pokémon X & Y do not have all of the Pokémon, there are various bits that aren't unlocked until you have transferred a Pokémon over from Generation V games into Pokémon X & Y. Most of these are hinted about normally when you converse with the various characters in-game, but remain locked until you have transferred the Pokémon and brought each of the following Pokémon to each respective area.

#386 Deoxys

Location: Ambrette Town

Details:
In the Fossil Lab in Ambrette Town, in the north-west part of the lab you will find various Meteorites. Four of these meteorites will allow you to change Deoxys' form between its four forms: Normal, Attack, Defense and Speed Formes.
#492 Shaymin

Location: Snowbelle City

Details:
Within the Pokémon Center in Snowbelle City, you will find this girl. Normally, she talks about the Gracidea flower and the history behind it, but if you show her a Shaymin, she will be very grateful and will give you the Gracidea. This item can be used to change Shaymin into its Sky Forme if it is not frozen and it is between 8am and 5pm.
#641 Tornadus, #642 Thundurus, #645 Landorus

Location: Reflection Cave

Details:
Just west of the Shalour City exit of Reflection Cave, you will find a scientist who states that there are actually many more Reveal Glasses than the one that has been discovered in Unova. If you show her a Tornadus, Thundurus or Landorus, she will give you a Reveal Glass which can be used to change those Pokémon between their Incarnate and Therian Formes.
#646 Kyurem

Location: Kiloude City

Details:
In the north-westernmost house in Kiloude City, you will find this Punk Girl who dreams of seeing a Kyurem. If you show her one, she will give you the special DNA Splicers item. This item is a key item that can fuse Kyurem with either Zekrom or Reshiram to create Black Kyurem or White Kyurem respectively.
#647 Keldeo, #648 Meloetta

Location: Snowbelle City

Details:
In Snowbelle City, in the house above the Pokémon Center, this gentleman is a part of a long line of family who have been raised to teach special moves to certain Pokémon. If you bring him a Keldeo, he will teach it the special move, Secret Sword, which will change its form into Resolute Form. If you bring him a Meloetta, he will teach it the move Relic Song
#649 Genesect

Location: Ambrette Town

Details:
In the Fossil Lab in Ambrette Town, you will be able to speak to this hiker, who asks to see a Genesect. If you show him the Genesect, he will give you the Blaze Drive, Chill Drive, Douse Drive and Shock Drive items, which allow Genesect to use different types of the move Techno Blast.