Well I won't touch on HG/SS because they are remakes of games we already knew and loved, plus we also knew exactly what would be happening.
Now let be get straight to the point.
What disappointed me most about D/P/Pt?
There are are couple of things really. I'm not saying I didn't like gen 4, quiet the opposite in fact.
The color pallet was abysmal, so many gray tones :/
The difficulty was laughable at best. But to be real, the only really "Hard" official Pokémon games are BW2.
So what they should do is implementing a hard mode. (I know they technically have tried, but their so called hard mode didn't make sense, or at least not the way they implemented it)
Is there anything else? Not that I can think of, it was the first game to have a good story that wasn't too convoluted, which of course made it a bit awkward at times, because of the lack of experience they had in writing an actual story for a main franchise Pokémon game. But it was good none the less.
They also introduced some of my absolute favorite Pokes, AND they implemented the possibility for a good competitive scene. (Let's not mention evasion, it's stupid no matter how you look at it....)