Growing the Beard: The first book in the series was arguably less imaginative than later entries, as most of the images featured a spattering of random objects over a two-dimensional space. In subsequent books, the photographs often contained entire miniature worlds, which, besides having substantially greater appeal for the imagination, used depth and perspective to heighten the challenge.
Surprise Difficulty: Some people might be surprised that, in spite of the target audience being children, the riddles can be shockingly difficult and even stump many of the parents who were trying to solve the riddles with their kids, particularly the ones involving homonyms, as noted by Brutalmoose.
Visual Effects of Awesome: Yes, in a children's book series. The grand majority of the books were written before Walter Wick moved to digital photography...which means that, yes, every single picture was completely real and done by hand, with zero photo editing. He even has a series of behind the scenes features on his web site detailing how he put together quite a few of the harder-to-explain pictures.
The video games:
Creepy Awesome: Skelly from the Spooky Mansion games, speaks entirely in rhyme, has a big, spooky house, created a series of riddles for the player to solve before being able to leave, and made a machine to create ghosts from simple objects.
Creepy Cute: The ghosts from Spooky Mansion. Tick-Tock and Meow in particular.
Growing the Beard: Spooky Mansion shaped the games into being themed like the books after I Spy, I Spy Junior, and I Spy School Days. Treasure Hunt then went onto to format the "three adventures" gameplay.
It's Short, So It Sucks!: The original Spooky Mansion spurred a lot of complaints of this, as evidenced by Amazon reviews. The Deluxe version alleviated this heavily by giving it an overhaul described on the main page; given that most people seem to remember it a bit more fondly than the original, it probably worked.
Critter, the ghost made from a snake, a frog, two flies, and an eyeball. No doubt he's made from creepy ingredients, but he looks so cute with his big eye.
The friendly aliens from I Spy Fantasy, also with big, innocent eyes.
Viewer Gender Confusion: Skelly from Spooky Mansion is referred to with male pronouns on the back of the Wii version's CD case and on Scholastic's website, but is often mistaken for a girl due to his androgynous voice.