(2) Sea Level
If there were a Best Friends to Readers Everywhere award, Pratchett would win it, hands down, year after year, and every year for the foreseeable future. Seriously—it would probably get to the point that he'd be excluded from the competition to be fair to all the other writers out there.
His writing is smart and intelligent without ever stooping to such cheap tricks as dizzying sentence structure and flamboyant word choice. Concepts as foreign to Earth-bound minds as multidimensional lollygagging and the addition of wavelengths to our vanilla light spectrum become as easy to digest as a Monty Python sketch.
With that said, like the difficulty of a video game, The Color of Magic does have a harder setting you can select if you wish. Consider this example:
Water on the Disc has an uncommon fourth state, caused by intense heat combined with the strange dessicating effects of octarine light; it dehydrates, leaving a silvery residue like free-flowing sand through which a well-designed hull can glide with ease. The Dehydrated Ocean is a strange place, but not so strange as its fish. (4.3.6)
Now you could read this passage and just enjoy it for its humor. But alternatively, you can read this passage and dissect it for all its scientific goodness, considering how Pratchett uses our physics to construct the odd, yet oddly similar, physics of the Disc. In this way, the Disc's wibbly-wobbly physics turn around and teach us about our own. It makes the novel a slightly more difficult read, but no less enjoyable for the extra effort.
And if you're into that sort of thing, might we suggest The Science of Discworld—a part novel, part scientific textbook that breaks down the science of the Disc to teach us about our own universe. The word awesome doesn't quite do it justice, but it's a good start.