Also, both nostalgia-tear-inducing opening themes.
Ear Worm: ''Trolls, and wizards, and fairy kings! Birds that talk, and fish that sing!..."
Fridge Horror: It's established that a gnome couple only have children once in their lifetime, and it's a set of twins. Simple math indicates their numbers will never grow and every gnome who dies before having their own pair of children forever reduces the Gnome population by one.
David and Lisa's granddaughter, Susan, appears in a number of episodes but her twin is nowhere to be seen. Who's to say the twin didn't die young?
Plot Hole: it has been mentioned several times that when the Gnomes enemies the trolls are exposed directly to sunlight, they turn to stone. However, in a lot of episodes the 3 recurring trolls are out in broad daylight and have no ill effects come over them.