Captain Picard's log entry states that they are bypassing their scheduled archaeological survey of Camus II. Camus II is the planet visited by Enterprise in the 79th (and final) episode of Star Trek (1966), "Turnabout Intruder (1969)." "Legacy" is Star Trek: The Next Generation's 80th episode.
As the 80th episode, this pushed 'Star Trek: The Next Generation' beyond the 79 episodes of 'Star Trek: The Original Series', unless one counts the unaired pilot "The Cage."
The Turkana underground complex sets are redresses of the Borg ship interiors.
The camera catches Data's poker hand. He has a potential straight draw, holding Ace-Deuce-Trey-Four-King, but no made hand. Rather than drawing, he opts to bluff, doing so successfully.
Hayne tells Picard "The enemy of my enemy is my friend". In Star Trek Into Darkness (2013), James T. Kirk also tells Spock this to justify his teaming up with John Harrison, a.k.a. Khan Noonien Singh.