I'm definitely a fan of Twinbrook, and I wish I could put it concisely, but I can't.
I first fell in love with the place for its scenery. Sunset Valley is a good town for building, but the constant cheerful greenery got boring quickly. Here was Twinbrook: a land of dark swamps, dead trees, and native land of the fog emitter. I love my worlds to look somewhat sinister like Twinbrook is. I sometimes just hang around to take screenshots of the scenery; the sunset over the rivers, nighttime fog over the swamps, ghosts rising out of their graves over the fog of the cemetery.
It was also the first town where I didn't have the urge to annihilate all the pre-mades. Even though my first play of Twinbrook involved clearing out everything, I ended up saving some families. They felt real to me. Even the rich families, such as the Bakers, the Pidgins, and the Rackets have a human air to them. Eventually, I kept everyone. Each sim seems to be crafted to feel like someone you might know, yet still keeping the exaggerated, wacky feel of The Sims. Sometimes you just have to play them to realize that the developers took pains to flesh out everyone in some way.
Lastly, I hate sims that look generic, and the natives of Twinbrook are just the opposite. Some sliders go to their extremes, and while the result isn't visually pleasing, it keeps the gene pool strange and strong. Plus, there's only a couple sims there that are ugly beyond all hope.