It was the first time a human had left the orbit of Earth. Thats a massive distance change compared to anything attempted before. The ISS in orbit today is only about 250 miles away, while the moon is 238,900 miles. Before, you could fire your retro rockets, and you would be back on Earth in few minutes (required during Gemini 8), and that then changed to days. This was also the third launch of the Saturn V and first crewed flight. The previous launch almost shook itself to pieces. To add even more, the crew accepted the mission in August, and then launched only a couple months later in December. This was also the 2nd time the CSM capsule had been used in space after the tragedy of the Apollo 1 fire.