T'Pol's disease, contracted in season one's "Fusion", is finally cured by T'Pau.

V'Las may have in fact been a Romulan spy the whole time, like T'Pel in the TNG episode "Data's Day".

We see Archer using the Vulcan neck pinch on a security force member, hopefully the removal of Surak's Katra won't affect this useful skill.

Features the first appearance of a Romulan, and the first hint of the upcoming Romulan War.

We learn that the Andorians have transporter technology. So far we know that humans, Klingons, Xindi (and probably Vulcans) do as well.