These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Nightmare Fuel: Neville going into a darkened building, a likely place to find lots of Infected, to search for his dog. He's so obviously terrified you can't help but be frightened with him.
The zombies look very frightening
One-Scene Wonder: She has about 1 minute and a half of screen time at the beginning of the film (and isn't listed in the credits), but Emma Thompson as Alice Krippin is so delightful you desperately want to know more about the woman who inadvertently created The Virus.
Word of God said that they were trying to use extras at first, but since it required constant hiperventilation, they were forced to use CG instead.
Tear Jerker: The aforementioned Shoot The Dog moment, and the moment Dr. Neville begs the mannequin to say hello to him. Though some found the latter Narmy. Also the "Look Daddy, a butterfly," line right before Dr. Neville's demise and the death itself. Finally the infected mother begging for her daughter to be taken to safety.
They Wasted a Perfectly Good Plot: The film's alternate ending comes closer to the message of the novel than the ending that made it into the movie and fills the above-mentioned plot hole, in addition to many others. It did poorly with test audiences, though, so Executive Meddling changed it to an ending that is completely different and misses the point of the entire novel, twisting the message so that it can still fit the title, but in almost the exact opposite way that the original ending fit it.