Spoiler Level: High
This is it, this is the one that changes the face of Star Trek. This is when we meet the Borg.
It's great to watch it in retrospect, knowing all we do about the Borg now. Originally we were seeing the events unfold from the perspective of the Enterprise crew-- not knowing what's out there, but confident in our heroes' ability to handle it. But after 21 years or so, we're now seeing events from the perspective of Guinan and Q-- we know the Borg, we know how unstoppable they are, and we know the crew has no idea what they're getting in to. When Picard walks up to his first drone and tries to begin a discussion, it seemed perfectly rational when the episode first aired-- watching it now, we know how ridiculously futile the gesture is.
A lot of things still hold up really well about this episode with the overall Trek mythos. Guinan says the Borg destroyed her homeworld a century ago, which tracks perfectly with Star Trek Generations. Q states that Guinan is centuries old, which we'll learn is true in "Times Arrow." And the Borg themselves are just as creepy as any later appearance, arguably even more so-- without the Borg Queen to speak for them, they're simply an unstoppable hive mind.
Not that everything tracks perfectly, mind you. Guinan says she wasn't there for the actual destruction of her homeworld. While her being on a refugee ship in Generations makes me assume she had been, if you're away from home when it's destroyed you still become a refugee. The voice of the Collective still sounds like a monotone voice, but it doesn't sound as much like a chorus as it would later. And then there's the whole rivalry between Guinan and Q, which never really was explained. There's never anything stating that Q has sent the Enterprise to the Delta Quadrant, like I had assumed there was-- where did that come from, anyway? If anything, they're only two years from the nearest Starbase at maximum warp, instead of the 70 years Voyager was, which confirms that the Borg had always been closer to home.
And while Q is the catalyst-- he's the one who gets to be the voice for the Borg, as the Enterprise's taunting guide-- he can't really have made that huge a difference in how soon the Federation encountered the Borg, since the Borg had already scooped up the colonies in the Neutral Zone.
I had read in Star Trek: The Magazine (which was a magazine I loved, but its title always made me laugh, because calling it The Magazine sounded like something right out of Spaceballs. But I digress. The magazine had an interview in it that said...) that Maurice Hurley, who wrote this episode, intended for the Borg and Q to have a long history together and be something of rivals, and their stories were to be constantly intertwined on a larger scale of good and evil. Then he left the show and that idea went with him, with the Q and the Borg taking totally separate routes in Trek history.
Something else interesting I discovered from rewatching this episode-- the Borg emblem is actually used. I first saw it with the Borg action figure by Playmates, and I thought to myself, where did they get that from? The Borg don't need symbols. Symbols are irrelevant. I figured Playmates had just made it up. Then when Lore had his group of emotional Borg in "Descent" they used the symbol, so I figured it was something the art department had come up with, given to the toy company, and it never made it on screen until then.
Well, I was wrong on all counts. Here it is on the wall in this episode. I've highlighted it in the screen capture. I guess symbols aren't irrelevant.
And last but not least, in contrast with the darkness of the Borg we have the introduction of Ensign Sonya Gomez, an engineer who's enthusiastic, chatty, bubbly, and a bit wet behind the ears. La Forge mentions that she came aboard on Starbase 173, which we saw the Enterprise visit seven episodes ago in "The Measure of a Man," so that's a nice little continuity touch. Ensign Gomez only got to appear in two episodes, but she did go on to become the star in the eBook series Starfleet Corps of Engineers (aka S.C.E. and later Corps of Engineers).