Fairy Types! Immune to Dragon, which nerfs the dragons considerably.
Fairy Type Moves! Moonblast with 90 BP and Draining Kiss recovering 75% of damage delt is HUGE.
Mega Evolutions! Some Pokemon become Digimon and evolve while in battle to a stronger form. Banette, Ampharos, Pinsir and more Pokemon can Mega Evolve.
Steel Resistance Alteration! Steel Types no longer resist Ghost and Dark, but resist Psychic now.
Certain Move Immunities! Grass Types now cannot be affected by spore moves, Ghost Types aren't affected by Mean Look, and Electric Types can't be paralyzed.
New Abilities! Abilities such as Refrigerate and Pixilate change Normal Type moves to Ice and Fairy types respectively, and Gale Wings gives +1 Priority to all Flying Moves.
New Pokemon! Get familiar with Pokemon like Trevenant, Aegislash and Talonflame, and because they are now wrecking the OU scene.
Stat Changes! Raichu got faster, Alakazam has more SpDef, and Scolipede has more Attack. Pokemon have recieved stat buffs, but no stat drops. Look at them before raging when an Alakazam is left with 1 HP after you use Dark Pulse on it.
Weather Nerf! Drizzle, Drought, Snow Warning and Sand Stream are no longer permanent! No more DrizSwim obliteration!