I specifically like
>The value in backtracking
So you get a new HM, or the story progresses a bit and some Trainers have new teams, or you reach the post-game and get access to dual slot mode, swarms, the Poké Radar, Super Rod, or general area enhancements like the Trophy Garden or Great Marsh.
>The post-game content
The second half of Victory Road, Route 224, Snowpoint Temple, Fullmoon Island, Spring Path/Sendoff Spring/Turnback Cave, Pal Park, the Battle Zone (Route 225, 226, 227, 228, 229, 230, Resort Area, Fight Area, Survival Area, Stark Mountain), Battle Tower in DP/Battle Frontier in Platinum, Rotom in DP, the Legendary birds in Platinum
>The events
The Manaphy egg from Ranger, Members Card and Newmoon Island with Darkrai, Oak's Letter and Seabreak Path/Flower Paradise with Shaymin, the Azure Flute and Hall of Origin with Arceus, the Secret Key with the Rotom appliances in Platinum, the Regis in Platinum, and even minor stuff like the event Pokétch apps that are all currently accessible in an authentic manner with DNS exploits (though I used an Action Replay back then).
>The plot feels like a big deal but doesn't railroad the game like later games
It never stops feeling like my adventure. The stuff with Team Galactic remains secondary much like Team Rocket and you don't feel like your hand is being held the entire way like with Lillie or even Cheren/Bianca or your friends in XY. You'll encounter Barry, Rowan's assistant, Cynthia, Cyrus, and Team Galactic at different points, but not a lot of it feels like one constant flow of story, more like isolated incidents like Team Rocket in Mt. Moon.
>The value in going out of your way
You get rewarded if you do stuff like carry Pokémon with HMs and take the time to explore, finding stuff like TMs or Trainers with rare Pokémon that are then added to your Pokédex. You'll find Wayward Cave or Fuego Ironworks or fully explore the Solaceon Ruins and maybe fully extend the path, etc.
