The most important new mechanic?

I think physical/special split introduced in the 4th generation was really imporant, just look at Gengar, It has a great special attack but in gen 3 ghost and poison types were phsysical.

Natures, Abilities, and the aforementioned Physical/Special split. They've honestly really shaped the competitive community to be the way it is! Them and of course the change of DVs to IVs, and the introduction of EV training, though I'm pretty sure it's been around since the start. x'D

Shinies though. Shiny Pokémon are really the kicker for me. Love those sparkles. <33

Physical/Special split is really important for me, it added an extra element to battling and gave different types their own advantages. Abilities are important and play a major part in the games too but I think the physical/special split changed the way we play the games so much.

Horde battles. It makes it easier to EV train and easier to find a Shiny.

Easily the physical/special split. I remember being so confused for the longest time. <_<'

The physical/special split without a doubt. Nice to see virtually everyone here thinks too.

I see it not so much as being a 'new mechanic', but more so as something that needed to be there from the start but wasn't implemented for some reason. It's analogous to fixing a huge bug that we all just went along for three generations.

To everyone that said the physical special split... you win! I agree. Another thing that maybe isn't considered a mechanic but infinite TM usage. Oh yes.

Thinking on it, I agree. Physical Special Split was probably the most major thing introduced. Abilities in general made a much subtler change, and natures are even subtler. However, the most "in your face" was the split.

Actually, in terms of the story game, I'd say that the infinite bag is most useful. I played the original Gen I games and I was shocked, so whenever the Infinite Bag came along was a great, yet I think uncelebrated, moment in Pokemon's History.

I personally think TM usage & the Special Split were very good features but I'm going to be different and sayyyyy

Natures.

Yes, natures can get a bit tedious time to time, but they give the Pokémon more of an unique feature to it then just being another ol' zubat or something. :)