Trivia: Have a close look at the Predator's trophy collection on his spaceship. Taking pride of place is a skull from the creature in the Alien movies. (01:33:05)
Revealing mistake: When Danny falls down from the ceiling, watch closely and you can see the security wire attached to his leg. (00:34:40)
Visible crew/equipment: Near the beginning of the film when the police car explodes and flips over, you can blatantly see stunt wires used to aid in flipping the car. (00:02:45)
Continuity mistake: In the scene on the subway before the Predator attacks. Bill Paxton's laser sight alternates being on and off every time the shot changes. (00:54:50)
Continuity mistake: Notice King Willies teeth before he is killed. They are nasty looking. After the predator cleans up his skull, the teeth are white and shiny and also change size. (00:47:00 - 00:49:00)
Continuity mistake: At the warehouse, when Harrigan and the other officers are looking at the carnage caused by the Predator, Danny points out the dead Columbian hanging from the skylight. The camera cuts to show the man hanging dead from the skylight, and you can see that he has a good amount of blood running down his body. When Harrigan is ordered downstairs, the scene cuts again to the body being hauled up by the Predator, except now the amount of blood has increased tenfold, looking almost like someone threw a bucket of blood on the corpse. (00:13:40)
Visible crew/equipment: When the Predator is attacking the Jamaicans in the penthouse, you will see the guy who gets pinned to the wall with the net propelled backwards. We are led to believe that it is the force of the Predator's weapon - not so - it is the bungie cable attached to his trousers. (00:24:55)
Answer: The predators are honour bound hunters, and as such they will only attack a target if it is capable of defending itself (i.e, if it is armed, which is why it doesn't attack the boy in the cemetary). Although the police outside were armed as well, the Predator presumably attacked the drug dealers because they were a harder target (as the police were outside he could have just sniped them from the roof tops, but he had to get relatively close inside the building) and as such, killing them gave a higher honour. The obsession with Danny Glover's character is probably because he is quite a brave and heroic person, similar to a Predator (the way he saves the other cop by risking himself etc.), so the predator may have been studying him for a while. There's also more honour in killing someone like that (as he'd be a hard target) than there is in just killing a random armed guy.
Gary O'Reilly