Did you know there's a guy who tells you what your pokemons hidden power is?
He's in Velistone in the prize corner.

Is there anything else that I'm missing from this game?

There's a guy in the Battle Tower that'll give you an idea about your pokemons IVs and it's best stat.

Didn't know about the HP guy, though.

Woah I didn't know that.
But I breed in diamond...

hey did you know that when you unlock the arceus evnt, you go to ireburgh mine, meet the fat guy, then you go to the library, you talk to him, you go down the stairs and thre is a guy downstairs that if you talk to him he tells you that he will meet you at the pokemon leage, and i think that he becomes the new champion becuse cynthia goes to johto so the arceus event in HG/SS can be done

Well, there's the villa, which you might have found. And several places have been changed to tutor places who tutor moves for shards. You can also meet Lucian from the Elite Four in the Canalave Library, Cynthia in the Celestic Ruins and other stuff.

On my platinum, the Power anklet in the Battle frontier stand that lets you buy it says
"A Pokemon hold item that promotes Speed gain on leveling, but reduces the Speed Stat."

Whoa. He tells you your Pokémon's HP! That's sweet.

the hp dude is awesome!
in the big coal hill in oreburgh gives you a heartscale and a pearl

There's hidden items all over the place, if you look with the itemfinding app on your poketch. I especially found a lot on the beaches.

lmao i didnt even know that

Not sure if many people know, but if you go talk to the kid with glasses at the battle tower you can make your own victory, loss quote in Wi-Fi battles.

I never knew that Riley was optional in the game but i would have done it anyway

Everyones probably noticed this, but in the dining room of the old chateau, when you walk in, sometimes the ghost of a butler will appear. If you walk in to the room with the eyes on the wall, the ghost of a little girl will glide out of the room.