I think 6th gen introduced many news about game mechanics, the megas, primals, the pixie type and some moves review.
I liked megas, i think they gave a new breath to the previus metagame, but i can't say the same for the primals, imo the new primal Groudon ability ruined the uber metagame, focus teams mainly around Primal Ground. I really suffered the difference between XY and ORAS.
Pixie type imo was a necessary type in order to nerf dragon types (onestly in BW2 dragons were too op), and to give more utility to steel and poison moves (before the introdiction of pixie those 2 types were usually discared for types who provided a better coverage), but despite this idk i didn't like it so much, i think they could think something better to nerf the dragon type.
Speaking about moves, defog was the best news. Finally something that allows u to remove hazards. That's why 4th and 5th generations were too based on set as much as spikes/tspikes/rocks, then keep a ghost type (mainly Giratina or Arceus) to stop rapid spin. Now with defog finally the game became more strategical, set rocks and spikes or stickywebs, another important news of 6th gen, it's still important but now it's up to player keep them alive using taunters predicting the opponent defogger switch in.
The 7th gen instead ruined everything, imo. As GS BALL said, they did too many nerfs, i personally really hated gengar ability change, that really doesn't make a sense....