In Pokemon Diamond, Pearl, and Platinum, you can speak with Prof. Rowan's assistant to get a special birthday message! (That assumes that the DS's date equates to the birthday set on the same.)
When switching Pokemon in all games prior to Pokemon Platinum, the game would say "Will [player name] change Pokemon?" Pokemon Platinum choose the dialogue "Will you switch your Pokemon?" in lieu of this, and this change has remained the standard since.
Regigigas can now be found in this game at Level 1, making it the lowest-level non-glitch Pokemon possible to be found (tied with Magikarp), and therefore also the lowest-level non-glitch legendary Pokemon possible.
In Pokemon Diamond, Pearl, and Platinum, there is an area called the Hall of Origin that most don't know about: it is an area that is above Mt. Sinnoh. Game Freak was supposed to hand out an item relating to this event known as the Azure Flute: this would allow the stairs to appear to the Hall of Origin and you thereby meet Arceus, the supposed God of all Pokemon (for he created the universe according to Sinnoh myth). However, this actual giveaway of the Azure Flute never occurred: every legitimate Arceus you'll see are from special direct-download giveaways that don't trigger this event that, although programmed into the game, never occurred. (And likely never will: by the time this was written, it has been over 7 years since D/P/Pt's release.)
The GTS feature in Pokemon Platinum allowed for e-mail notifications of trades in the Japanese version, although this was downgraded to notification through the Wii Message Board for international releases.
Interestingly, Pokemon Platinum runs at 30 frames per second rather than 60 frames per second most games tend to use.
The manual for Pokemon Platinum notes that there are 493 Pokemon to be found: however, this was before the reveal of Arceus, #493 in the National Pokedex, which left many to wonder as to what #493 was as everything up to #492 had been revealed in some way.
Funnily enough, in Pokemon Diamond, Pearl, Platinum, HeartGold, and SoulSilver, Kadabra will still evolve into Alakazam when traded, even if holding an Everstone. An Everstone is supposed to prevent evolution, and it does with all other evolutions except that of Alakazam for unknown reasons.
To date, Pokemon Platinum is the only expansion version of a Pokemon title to be released in South Korea.
In the Sinnoh Pokemon games, when doing the event in which you obtain a Manaphy Egg, this Egg is normally Shiny Locked - it cannot be Shiny. However, if you trade the Egg to someone else, who then hatches it themselves, the Egg does have the usual 1/8192 chance to be Shiny. This is currently the only way to legitimately obtain Shiny Manaphy.
The years in which Pokemon Diamond and Pearl were released were also the introduction of the Pokemon VGCs, or "video game championships": these were competitions held at the regional, then national and global levels in which people would be able to compete directly with other players for the title of Pokemon World Champion. This in turn began to make players more concerned about factors normally trivial for non-competitive play, such as EVs and Natures, and was also the rise of a number of battle simulators to lessen the amount of work on people that would possibly be spent in making a flawed team.
When the games of this Generation of Pokémon introduced the Physical-Special split for Moves' damage categorization, Fighting and Psychic were the only two Types that saw none of their previously existing (damaging) Moves change category: all of the old Fighting Moves were Physical before and remained Physical even after the split and all of the old Psychic Moves were Special before and remained Special after the split.
When the games of this Generation of Pokémon introduced the Physical-Special split for Moves' damage categorization, Dark was the only Type that saw all of its previously existing (damaging) Moves change category, since they all became Physical whereas they were all Special before the split.
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