First off, we've done a name change from Smash Bros. Legacy M 3.6 so it was less confusing with the version number. Super Smash Bros. Legacy is based upon Project M 3.6, so we don't need to add that in the title. "Smash Bros. Legacy" is the name of the dev team, sans "Super."
Files included are hackless, homebrew, portable netplay, and sd.raw versions. There are also a "hd pack" and "music dlc" files as well. Portable netplay includes Dolphin, a for-netplay sd.raw file, and the entire HD pack.
We've had standardized portraits since 1.2, but now we've posed and rendered each costume ourselves for new CSPs and BPs! Not only that, but with the HD pack, you can experience it on Dolphin with beautiful high-resolution textures.
We've added some new skins, stages, and songs, but most of all we've added Rainbow mode and a Retro Mode (currently in alpha). Rainbow mode is accessible by holding down Z on the Character Selection Screen, while Retro mode is accessible in Special Mode.
The music DLC pack is essentially all of the songs from 1.21 we had to remove from 1.3 so it took up less file space. Since adding every song is over 2GB, you'll have to pick and choose which songs you want from the music pack unless you are able to use storage media that allows for over 2GB storage capacity.
Lastly, be sure to check out the credits.txt that's part of the download. The folks responsible for the amazing mods in this pack are listed there. Also secrets like how to get to some of the stages in the trailer above (rainbow road stage, Dark Link room stage, etc)!