Something I'm rather surprised to find that no one has mentioned, Sun and Moon appear to be doing away with the series staple of HMs, through the use of a new system called Pokéride.

Essentially the way these work is, you get a handful of Pokémon at your beck and call throughout your journey. These Pokémon do not join your team and will not battle for you, but can be called to serve a purpose on the overworld at any time.

Known Pokémon for this feature include:
Charizard - Fly and possibly Soar.
Lapras - It's a Lapras. It Surfs.
Sharpedo - Faster Surfing, and the ability to break through rocks in the water and jump.
Tauros - Fast movement on land (maybe replacing bicycle?), can break blocks.
Stoutland - Movement on land, can sniff out hidden items.
Mudsdale - Movement over rocky terrain.

Details aren't fully known. Will Lapras be useless once you get the faster Sharpedo? Or will it be needed for deep water between islands? The full extent of how many there are is still a mystery as well. But given the number of confirmed field uses, I wouldn't be surprised to see them taking the place of the all too familiar HMs.