That's a tough question -- separating extreme but predictable developments from ones you never saw coming.
My choices are:
1) The social power of the internet. If you'd asked me in 1992 what the internet would be, I'd have predicted it would never be anything but shopping, porn, and information. I never thought people would live as much of their lives through it as they do today.
2) The political polarization of the US around religious conservative issues. The religious right was around for so long (10-15 yrs at least) without really being a dominant force, that by 1990 I would have predicted it never would gain any more power. All of a sudden, after the 1992 election, the religious right (the "base" of today) just exploded into control of the Republican party and now defines the major political/social divide in America.
3) Anime. When I was little, Japanimation was cheap, cheezy, low-value afternoon programming for schooltykes (God I hate the censor). "Real" cartoons were Disney, and the Warner Bros stuff from the 1940s and 1950s. Even "new" stuff like Pink Panther, and Rocky and Bullwinkle was made in the 1960s. I still am blown away that Japanimation is now the dominant style of comic illustration, cartooning, and movie animation. What was originally a cheap alternative to traditional animation has become the preferred style of 2 generations -- talk about making a silk purse from a sow's ear!
-mid