How would you characterize the Fantastic Four (both individually and as a group) to someone who first entered the world of comics after Marvel phased them out? What exactly was their niche prior to Secret Wars?

I would tell them that the Fantastic Four were the laboratory of Marvel Comics: they came first, the initial experiment in inter-personal dynamics and flawed characters was exported to the rest of the Marvel Universe (indeed, the idea of the Marvel Universe started there), they were the place where you could explore the weirdest corners of the medium (both in terms of writing and art), and finally they were a font of brilliant characters that could be used outside the FF (Doom, Kang, Galactus, etc. etc.).

In terms of their individual characters, they fit pretty easily into the four humors model (although man do people love to argue about who fits where): Johnny is a playboy and a prankster, Ben Grimm is depressed and has a hair-trigger temper, Reed is the A-type scientist/leader/authority figure, and Sue Storm holds the universe together.