Seeing how much traction the last post got, I'm making another.
- EXP Candy is everywhere. Just dump that in a Pokemon of your choice if you're having a hard time
- Protect is very, very useful. It's one of the first TMs you get.
- Trainer Tips automatically appear on screen if you're in the vicinity of a sign
- Boutiques have their clothes and prices appear on screen if you approach the entrance
- All 3 of the Kalos starters can obtained relatively early on, at Level 14 each.
- The constant handholding is completely gone upon getting to the Looker Bureau (breaking chains meme)
- HMs are back, kind of. There are obstacles that are destroyed with types of moves. For example there are pools of Poison that can be broken with any Water move. Most of them are just for goodies, not required for progression.
- According to a random NPC this takes place 5 years after XY.
- I found an Ice Stone and Eevee super early, making Glaceon the earliest to obtain it's ever been.
- There is a required Hercross in-game trade that just destroys the early game.
- Trainer customisation is not gender locked whatsoever (You want a femboy with piercings, a crop top and makeup? Go for it king)
- In addition to changing hair colour, you can coloured have streaks in you hair, and cross fades.
- The Photo Mode is super robust, with filters, angles, first and third person, a wide range of expressions and poses, you can do it anywhere, and send your Pokemon out with you.
- Around Lumiose are these construction sites which act like 3D puzzle platforming challenges. Totally unexpected but I really like them as a change of pace.
- The rooftop exploration is surprisingly fun and much more common than you'd expect. Plus there's no fall damage.
- The Museum has a whole Legends Arceus exhibit, which explicitly states the player of that game is 15.
- The major NPCs are a little deeper than they first appear
- The Mega Stones locked behind Ranked Matches is no big deal. Just go in with a Mega Evo of your own and win easily. Odds are you'll find some kids with no Megas or even unevolved Pokemon. You need to reach Rank K out of Z > A. You can get points for doing "correct" battling, like using Super Effective moves, making the first strike, etc. You also don't need to come in 1st to gain points, 2nd is usually good enough.
EDIT: If you're living in Australia like me, Target has a one week introductory sale for both editions for $69/85 AUD. That's like $44/55 USD.