All sorts of goodies in there. In addition to differing takes on various gen 2 Pokémon (including very different Legendary Beasts, an unrecognizable alternative to Bayleef, and a completely different take on Porygon 2 [the weird dog next to alternate Lickilicky]), there's alternate takes on Pokémon that showed up in later gens, such as Tangrowth, Mime Jr., Mega Pinsir, Lickilicky, and Leafeon, as well as alternate water starters that appear to have inspired the Popplio line.
It seems like at one point the plan was for all gen one two-stage evolutions to become three-stages, and all one stages to become at least two, though many were cut or used later. There's several entirely scrapped babies of gen 1 Pokémon, including Vulpix, Tangela, Goldeen, Paras, Doduo, Meowth, Ponyta, Grimer, and Growlithe. Girafarig also has a baby as well, in the form of that odd ghost-looking Pokémon next to it. Additionally, there was a scrapped Ditto evolution (still only learned Transform), a Farfetch'd evolution, an alternate evolution to Weepinbell, a la Bellossom, and a Quilfish evolution. The Shellder form from Slowbro and Slowking is also present as its own species.
Then there's the entirely new Pokémon that were dropped altogether, with seemingly no analogs in later games. A drastically different fire evolution line (previously known by its starting evolution via old screen shots), a full three-stage water evolutionary line (perhaps tied to the Lake of Rage), a two-stage cat family, a kinda dope fire seal named "Bombseeker", an electric tiger evolutionary duo, a pair of... voodoo doll things? (probably a reference to something, if someone wants to help me out), and a duo that as far as I can tell appears to be wearing another Pokémon's skin? lol. Almost reminds me of Mimikyu in concept.
So yeah, lots of interesting stuff there.
Next up is the world map, which had some big differences:
It appears that originally, Johto was intended to represent the entirety of Japan. Here's the complete rip of the map, which reveals some pretty extreme alterations:
Perhaps most interesting is the town at the bottom right, which may look familiar. It's the entirety of Kanto, compressed down into one very large town. This was likely GameFreak's somewhat desperate attempt to squeeze Kanto in while working around space restrictions, until Iwata famously saved the day and wrote them a compression tool that allowed them to restore nearly the entirety of Kanto.
There's a whole lot more here and here if you're interested.