There are some new mechanics:
Flare Tools: Head Ringer and Jamming Net are tool cards you can attach to your opponent's Pokemon that disadvantage the EX Pokemon it is attached to.
Spirit Links: These cards make Mega EX Pokemon playable as they allow you to not end your turn to Mega Evolve. They take up the tool spot of the Pokemon attached to though.
Lysandre's Trump Card: While you never actually saw it in action, this card is now banned.
Primal EX Pokemon: These are only Primal Kyogre-EX and Primal Groudon-EX. They literally mean the same thing as Mega Evolution, despite the naming of 'Primal Reversion' on the text of the card.
Ancient Traits: These are like abilities that some Pokemon possess that are like abilities, but aren't counted as abilities or effects of attacks (they are a whole 'nother separate mechanic). They can make your Pokemon attack twice, attach 2 energy, or even stop all effects of trainers done to that Pokemon.
Double Crisis: These are "Plasma"-like cards in a Aqua or Magma themed set. There's no real notable card in this set either then a bunch of cards that rely on Team Aqua/Magma Pokemon.
Skyfield: New stadium card that allows for 8 benched Pokemon in play.
Wally: Lets you evolve any Pokemon turn 1 (Not just Flashfire Butterfree line).
Some noticeable additions to the TCG: In Phantom Forces, Mega Pokemon got gold Secret Rare versions of the original card. Also, there was a Secret Rare Full Art Dialga-EX, the 3rd Full Art Secret Rare Pokemon (Reshiram + Zekrom in Legendary Treasures first). In Primal Clash, Mega Pokemon changed from Secret Rare gold editions to Regular Art and Full Art cards (not secret rare). In addition, the re-introduction of secret rare item cards starts back in this set with 4 secret rare items.
Some important Pokemon to look at: (Phantom Forces=PHF, Primal Clash=PRC, Roaring Skies=RSK) Manectric-EX and Mega Manectric-EX (PHF), Crobat line (PHF), Gengar-EX (PHF), Joltik, Lampent, and Pumkaboo (PHF), Malamar-EX (PHF), Dialga-EX (PHF), Heatran (PHF), Aegislash-EX (PHF), Florges-EX (PHF), Swampert Holo (PRC), Huntail (PRC), Kyogre-EX and Primal Kyogre-EX (PRC), Medicham (PRC), Groudon-EX and Primal Groudon-EX (PRC), Crawdaunt (PRC), Gardevoir-EX and Mega Gardevoir-EX (PRC), Bunnelby Ancient Trait (PRC), Ninjask and Shedninja (RSK), Banette (RSK), Absol (RSK), Togekiss (RSK), Latios-EX and Mega Latios-EX (RSK), Both Rayquaza-EX and both Mega Rayquaza-EX (RSK), Reshiram (RSK), Altaria (RSK), and last (but not least) Shaymin-EX (RSK).
Some important Trainer/Supporter/Stadium cards to look at: (Phantom Forces=PHF, Primal Clash=PRC, Roaring Skies=RSK): AZ (PHF), Battle Compressor (PHF), Dimension Valley (PHF), Enhanced Hammer Reprint (PHF), Head Ringer and Jamming Net (PHF), Robo Substitute (PHF), Roller Skates (PHF), Steel Shelter (PHF), VS SEEKER (PHF), Xerosic (PHF), Acro Bike (PRC), Archie's Ace in the Hole and Maxie's Hidden Ball Trick (PRC), Escape Rope and Rare Candy Reprint (PRC), Repeat Ball (PRC), Rough Seas (PRC), Silent Lab (PRC), Teammates (PRC), Mega Turbo (RSK), Revive Reprint (RSK), Skyfield (RSK), Trainer's Mail (RSK), Wally (RSK), Winona (RSK).
Some important Energy cards: Mystery Energy (PHF), Wonder Energy (PRC), Double Dragon Energy (RSK).
I hope this was helpful!