Several things which I constantly get PMs about:
1) The two 'new' natures which were noted from Japanese sites before the games' release - as far as I can see, neither of these natures exist, and I can't find any current Japanese sites that document them either. So most likely it was a rumour (or prank) posted by the Japanese websites during the pre-release time.
2) In the Pokemon Mansion, two maids block the left and right exits of the building, and there have been heavy speculations on where they lead to. Recently, one of the Japanese sites posted up a short video on a hacked game which had the maids removed, but those 'exits' aren't actually accessible even without someone blocking them. So the conclusion is they are only for decorative purposes.
3) There are have speculations on how Arseus's unknown variant was unlocked. Well, I've tried making it hold various either items, and none seem to trigger that form. I tried using moves that tamper with its ability (e.g. Skill Swap, Digestive Juice) during battles, and apparently, none of these moves can affect Arseus.
4) Feebas are caught in Tengan Mountain, in the mist-filled lake, and just like in RSE, there are only selected fishing spots in that lake where Feebas are found. I don't know the exact number of fishing spots though. Some say it's 6 like before, and some say there's only 1, since that lake isn't really that big.
5) By now you should have known that there are a total of 5 side characters who would pair up with you in specific locations along the game, during which even wild pokemons you encounter will be in double battles. Many wonder where they've gone since you don't seem to see them again. It turns out that in the Battle Tower, when you pick the double battle option, these 5 characters will be waiting to pair up with you in the Battle Salon, just like the Battle Tower in Emerald. The only difference is these people have actual backsprites now, so you won't have that weird front sprite VS front sprite battle screens.
6) Explanation on some of the unobtainable key items:
a) Red Chain - this is the item Team Galaxy created from the Legendary trio by chaining up their jewels. It lifts the seal on Dialga/Palkia on Tengan Mountain, and thus allows the boss of Team Galaxy to awaken those creatures. It is an in-game item alright, just not accessible to the player, like the Red and Blue Orbs in Emerald.
b) Volcanic Rock - when you do the Hard Mountain quest, the red haired girl Bagu snatches this item from the Hard Mountain, believing that by obtaining such a rare item in such a perilous area, she has proven herself to be strongest trainer in the world. Unfortunately, this act awakens Hiidoran. Again, another in-game item that the player isn't supposed to get.
c) Treasure Sack - this is actually an automatic function when you go underground. Guess this item was initially planned to hold the treasures, but Gamefreak changed their plans and simply made it an automatic option in the underground world.
d) Contest Pass - for some reason you don't need the Contest Pass to attend the super contests (actually, I don't think you need this either in Emerald), so this is likely another redundant key item.
e) Sticker Sack - actually a duplicate of the Sticker Box, possibly another redundant item.
f) Boat Ticket - this is probably the only inexplicatble item since it is described to be a ticket for S.S. Anne. Not sure on this one since S.S. Anne has not been referenced at all in D/P as far as I can recall.