This is the first time that third-party characters have been used in the Smash Bros. Series, meaning the first time they have added in characters not from Nintendo. These characters are Sonic the Hedgehog from SEGA's Sonic the Hedgehog series and Solid Snake from the Metal Gear Solid series.
When playing as Solid Snake in "Shadow Moses Island" if you make a "down taunt" very quickly you will activate Snake's codec, and a character from the Metal Gear Solid series (Col. Roy Campbell, Otacon or Mei Ling) will contact with Snake to give him information about the fighter or one of the fighters he is facing.
In the ice arena of the Pokemon Stadium there's a little shack that holds a picture of a white kitten. This is thought to be Masahiro Sakurai's kitten.
First Smash game to feature a character that is from a M rated game.
This is the first Super Smash Bros. title to feature the Smash Ball.