Message #101689

[Nyki]

Quite a few, because it's so early in the run.  It has the first spaceship to feature in the series, and the Sensorites are the first species that aren't purely human (like the Thals and the people of Marinus) but still humanoid (and no, the Voords don't count - they were meant to be humans in rubber suits).

Also the first description of what the planet later known as Gallifrey looks like.