If it wasn't for the Onslaught story line I would have never picked up an issue of Fantastic Four. After those Onslaught issues I followed the FF for years.
I can think of several times that has happened to me. I never read green lantern corps until the Sinestro Corps story. After that I picked it up religiously. Until that pt I could care less about the corps. I was content on reading my stories about Hal.
During the 90s the various xmen crossovers got me interested in several of the smaller x-men titles. I originally only read x-men and uncanny but after fatal attraction, I started reading X-force and X-factor.