I already talked about how good I think these will be in Legacy at length last week, but I just wanted to mention here that between fetchlands, Serum Visions, Sleight of Hand and Gitaxian Probe, Modern also has most of the tools necessary to enable them, so I expect to see a splash there too, though maybe a slightly smaller one
The card itself is a reference to a meme of Emrakul being taken down by 15 squirrels
Draft booster packs contain 15 randomly assorted cards from the set
A lot of the cards in the deck can make mana too - which means you can cast big game-ending spells like Tyrranax Rex and The World Spell when youve had enough fun and need to win
Then people move on to the next generation because once you've gone through Jungle, Base, and Fossil, you pretty much have one copy of every [first-generation] Pokmon
You can download the SIM client here