I personally don't get why this game is getting as much hate as it's getting. I think it's a great game! I've played Pokemon since Pokemon Platinum and, yeah, not much has changed from the original Diamond and Pearl but that doesn't make it automatically bad. If anything, I quite like that it kept most things the same.
I really love the art style they chose to go with. Of course, a fully proportioned Sinnoh would have been amazing but I still really like what they went with here. In a way, even though it's 3D, it feels very nostalgic and brings me right back to the days when I was a kid and I love that! Also, because the environment is more 'enclosed' rather than a more open experience, it gives off a real 'cozy' vibe and just gives BDSP a really relaxing feel to it. The Chibi models are also really good although a few characters do end up looking kinda goofy and unfitting (Cyrus for example) but for the most part looks really nice.
The story of the game is from my memory exactly the same as the original but that's fine with me because the story was great to begin with. I've noticed many fans complaining that they would've like to see more Platinum content and sure it would've have been cool to see Giratina during the story but again, I don't mind the end result because the story is still really good.
As for other things related to Platinum that aren't in this game, well, BDSP is supposed to be a remake off Diamond and Pearl, not Platinum. And the reason Platinum wasn't the one that got remade was because Platinum was the third version of gen 4. Kinda like Crystal to Gold and Silver or Emerald to Ruby and Sapphire, the remakes for these games have always focused on the main versions, not the 'more polished' version for lack of a better term.
Now for prices. I do agree that they're on the steeper side of things. I feel like they could have been half the price but that being said, if my memory is correct I think all past remakes of Pokemon games had been the same price as other mainline games at the time of their release.
The Grand Underground is what I think is the biggest change from the originals. In the old ones you couldn't find wild Pokemon in the underground but now you can. Even better, you can find Pokemon you never could have in the originals with transferring from older games like gen 3. This makes it much easier to fill out the Pokedex and get more varied options for teams. My only issue (and my biggest issue with the game as whole) is that your base's features are different. Instead of making a cool hideout similar to ORAS where you can decorate it with plants, plushes, beds etc... you now just litter it with statues. They have a fantastic mechanic don't get me wrong but I miss just having a little hideaway like in the originals.
The music. Goodness the music. Absolutely incredible. The moment I heard the town music I was transported back in time to Platinum. It is definitely one of the best parts of the game.
Finally, compared to the original Diamond and Pearl, I think BDSP is better. Not to say the old ones were bad, they were very good but BDSP is possibly a more polished and refreshed version. However, it doesn't compare to Platinum. That one will always reign supreme but BDSP is a really good game and deserve nearly as much hate as it is getting. Now it has it's flaws, every game does, but I do think it's a game many fans, old and new, can enjoy playing.