Which retailers have released the game early remains unknown, though what has been apparent is that their copies are the real deal. Additional footage of most gameplay elements has been made accessible now, with dataminers working hard to uncover various aspects of the game, including new items and the return of gameplay features like rematch teams.
It appears as if the vast majority of Gym Leader teams are their exact Diamond and Pearl versions instead of the more challenging Platinum rosters. After completing the main story, you may be able to rematch Gym Leaders, a feature not available in Diamond and Pearl. It’s possible that rematch teams may take influence from the Platinum teams. Sadly, the Sinnoh Pokédex is the measly 151 of the original games, as opposed to the expanded 210 from Platinum. Some of these Pokémon, as well as a couple of others, will be obtainable in the Grand Underground during the main game, just without Pokédex entries. Regional forms and evolutions of the available Pokémon, most notably Sylveon, are absent.
Despite the regional Pokédex being limited, all of the first 493 Pokémon are to be obtainable in some form, with Mew and Jirachi given to players with Let’s Go and Sword and Shield save data on their Switch systems respectively. Interestingly, you can get these two not long after the first Gym Badge, which would make any playthrough a whole lot easier.
The UI is pretty simplistic, but retains a lot of the details from the DS games. The colour scheme is close to identical to the original games, only modernised. In terms of graphics, the overworld is visually lacklustre, but the battles are a massive step up. Some visual glitches have been observed so far, like the textboxes with location names reflecting in the water, trainers looking bald due to their hats not loading, and battles happening in the overworld rather than the battle screens. Some of these graphical glitches could be due to people emulating the games on PC, as the ROM files have been made available.
Unfortunately, when it comes to the postgame, it appears as if fans were right to be worried.
The Battle Frontier has been cut, as it was in Omega Ruby and Alpha Sapphire. The Battle Tower is the only returning area of the five offered in Platinum, which was true in the original Diamond and Pearl as well. Multi battles have also been removed from the Battle Tower. Mega Evolutions are also nowhere to be seen, as I’m sure many expected due to the remakes’ emphasis on ‘faithfulness’. The Poké Radar is back at least, which will be good for Shiny hunting.
The Distortion World is also absent outside of a brief homage, which is a shame as it was one of the most memorable areas in Platinum. Likewise, Looker, who has appeared in most Pokémon games since his Platinum debut, is absent. It’s clear that these games are solely remakes of the initial two versions, unlike HeartGold and SoulSilver which were effectively remakes of Crystal rather than Gold and Silver. The few features from Platinum that they have chosen to retain, like the PC backgrounds and the expanded list of Pokémon that can follow you around Amity Square, are superficial and odd choices.
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It appears that the music for the games is currently a placeholder, as the quality is lower than what has been showcased in the official trailers. A patch will reportedly fix this. Weirdly, the 1.1.0 patch was released on the 11th of November, 8 days before the release date. There is also a DS Sounds item, which lets you play the original DS tracks if the patched tracks are not to your taste. The title screen animation was also missing, with patch notes confirming that it was added in the 1.1.0 patch. Access to various areas was restricted too, with the patch opening these up. This may become problematic for players that don’t have access to the internet.
Just as the game has been patched, so too have the modding community been able to get their claws into the game and change it up in various ways. Soundtrack changes have been implemented, and cheats and exploits are already being discovered before the game is officially released. With these games being made in Unity, it's possible that mods will be easier to develop than they ever have been for Pokémon games.
Pokémon Brilliant Diamond and Shining Pearl are intended to release on November 19th, though by this point many fans will have already fully explored their new take on the Sinnoh region, down to the last berry patch.