I'll do my best to run through the odd plot quickly. As I recall, Troll is a Harry Potter fan-fiction about a time in the 80's when a middle-aged man named Harry Potter (Michael Moriarty) moves his family into a new apartment building.
As it turns out, the building's full of little singing monsters...
...and naked Julia-Louis Dreyfuses...
Luckily, Potter's son, Harry "Atreju" Potter Jr. drops by to save the day with his golden spear and the help of Eunice, his elderly accomplice... but only after he gets beaten up by a little girl.
If that sounds awesome, it's because it is. Troll turns out to be much better than it has any right to be. The main reason is the great ensemble cast and the array of truly likable characters that they manage to portray here. I find most child actors to be almost unbearably bad, and the two kids are actually the main characters here, but these guys really do a fine job.
Julia Louis-Dreyfus appears here in her first feature-film role, as "pretty girl living in apartment" but does a great job of making her background character real in a way that a lesser film (especially in this genre) wouldn't have bothered to do.
The character of Torok the troll is taken in an interesting direction. He's not played for straight-comedy like the Leprechaun, but he's not really scary either... he just is. The only real conflict is that he and all the tenants of the apartment building must share the same space, but require different environments. The movie seems to go out of its way to make him not be scary actually... But since he does go ahead and turn people into trolls (or things that would better fit in troll-world), and that's probably a bad thing for those involved, the movie takes several measures to redeem him afterward. First he turns Malcolm into an elf in order to cure his illnesses and give him a chance at a life where he can be normal. As if that wasn't enough, when the time comes for Harry Potter Jr. to kill the giant monster at the end (and he fails), Torok goes ahead and kills him for him... thereby defeating himself. If that doesn't make any sense to you, don't worry, there's an easy explanation. At some point Torok must have simply decided that it would be easier to get a fresh set of clothes, change his name to Hoggle, and head out to the Labyrinth to work for David Bowie.
One last thing that I almost forgot is the troll chorus. It was composed by Richard Band, brother of the producers, but this whimsical tune could've easily come from Danny Elfman.
Troll is always more fun than I remember it being, and it's probably tame enough to be appropriate for even the youngest kids.
As an added bonus, the only DVD version I've ever come across is a double-feature disc that also includes the completely unrelated but wonderfully awful, Troll 2.