The episode starts with a nice misdirect in the form of Ensign Gomez, a clumsy proto-Barclay who is new to the ship. You expect the episode to be about someone integrating into the crew, but instead Q shows up.
While we never really got to know who or what Guinan was, her interactions with Q are always fun and she was interesting and 'powerful' in a different way. Here she is used mostly to deliver exposé, which works really well. Whoopi might be a terrible person but she did a fine job here.
The Borg themselves are fantastic. Sure, they are somewhat derivative of the Daleks and Cybermen from Doctor Who, but they transcend both. Some people think they represent the threat of Communism, with their lack of individuality. In the episode itself they are ironically called the ultimate consumers, who absorb who and what they want.
Their design is fantastic, with their weird ship and their odd implants. Having seen Voyager it is easy to forget how menacing they originally were, especially because they methodically study the Enterprise and practically ignore the crew - even when they're boarded! Now humans have become the new life that is sought out.