This was the first set released by Wizards of the Coast to include a "secret card", that is, a card whose number is larger than the set number, as well as a card created by Wizards themselves - Dark Raichu. Dark Raichu would not see a Japanese version until Neo 2.
The release of this set in North America was stalled considerably, as it was six months between when Fossil and Team Rocket were released. This mostly had to do with Wizards trying to get approval from Nintendo to make Dark Raichu; Nintendo was holding out and Wizards couldn't get the set released without Dark Raichu.
Similarly, this set was the first set to include the Japanese version of a "secret card" with the "Super Rare" rarity (signified by a white star), which was the Here Comes Team Rocket! Trainer card.
This was the first set to feature Dark Pokémon. For its North American release, the "Dark" prefix was chosen versus the originally translated prefix, "Bad", and the potentially touchy prefix, "Evil".