At least five of the Borg drones are Klingons, If Resistance Was Futile for them, what chance would the rest of us have?
The montage in which we see the Enterprise crew failing to hold back the Borg, during which we see several officers are being assimilated. One looking in horror as his amputated arm gets fitted with a cybernetic claw. Another with a freshly gouged eye being covered with an eyepiece.
The entire opening. We see Picard flashing back his time on the Borg Ship and assimilation (Eye Scream included). It's frighteningly disturbing seeing him plugged in to one of those rechargers with this completely emotionless expression on his face, and at this point he's still in uniform and hasn't even had any implants put in yet. And then these flashbacks finish and we cut to present day Picard looking upset after recalling all these traumatic events. Then he goes up and looks at himself in a mirror, and BOOM! - a Borg drill pops out of his face. Yes it turns out It Was All A Dream. It doesn't make things any less freaky to watch.
Dear god, the Borg were bad enough in the TV series, but anytime they showed up in this one the movie actually turned from a sci-fi film into a damn horror flick. One of the scariest scenes is where a handful of crewmen flee inside a darkened room on the ship... and then several Borg lights start flickering on within the room. It gets so bad that anytime we transition to Riker's party talking with Cochrane down on Earth it actually comes off as a welcome breather.
The warp coolant that dissolves (and seems to vaporize) tissue on contact. It's inexplicably kept in gaseous form in an easily-breakable vessel in main engineering.
To be fair, it's easily-breakable by Data, who has been shown to have Super Strength.
The blank looks and thoughtless eyes on the crewmembers' faces when they're in the early stages of assimilation - they're not afraid anymore...
The one crewman who begs Picard to help him as the assimilation process begins. It makes it so much worse when you realize these are probably his last conscious thoughts and feelings before his mind is superseded by the Collective.