Key Plot Twists: The most shocking moment of the series occurred in Season 1, Episode 23 ("Skin of Evil"), when main character Tasha Yar was abruptly killed, proving main characters were not safe. The Season 3 finale, "The Best of Both Worlds," saw Picard assimilated by the Borg, a twist that traumatized the character for the rest of his life (and into the Picard series).
The Finale's Meaning: In "All Good Things...", it is revealed that the trial of humanity by Q, which started in the pilot, never ended. The twist is that the exploration of space was merely a backdrop for the exploration of the mind. Picard realizes that understanding different possibilities (the three timelines) is the key to humanity's evolution. The series ends not with a bang, but with a quiet game of poker, signifying that the journey was about the people, not the stars.