Was the defining change of the 90s... the internet? It sounds so trite and oversold, but maybe because there's a kernel of truth in it. Maybe that really was the most important change of the decade.
I was talking to a friend who said he thought the 90s was the decade that had the most change of the past half century, and I was pretty surprised: to me, the 90s seemed very static and stable. The 80s, on the other hand, seem like a time of constant change and turmoil. Maybe perception about this is biased by what decade you grew up in?
Something that's been floating around for so long that I can't really attribute it is the idea that decades begin on defining moments that change everything. For example, right now it seems like the zeros definitively began on 11 Sep 2001: everything that happened in 2000 and the first half of 2001 was really just the trailing end of the 90s. The people I've discussed this with have generally agreed that the 90s began in Nov 1989 with the fall of the Berlin wall, and that the 80s began in Jan 1981 with the inauguration of Ronald Reagan.
Most people also agree that 22 Nov 1963 started the 60s, but there's not a lot of agreement on when the 70s started. My personal opinion is that they began when Nixon resigned, in Aug 1974. That means that some decades begin and end a few years before or after a strictly mathematical decade would, but since we're describing cultural shifts, I think that's okay. Non-Americans right now are grumbling that 3 of the 5 decade-changing events I mentioned were of interest only to Americans (all 3 involving presidents): global citizens are free to pick their own dates, but I think a lot of these events (especially Reagan's presidency) directly affected the entire world.
To me, the 80s changed everything: the personal computer, the huge resurgence of conservativism, drab earth colors replaced by bright primary colors. Tons of things got built. The global political climate changed dramatically. Music exploded from 2-3 genres to about 20. Space travel became so commonplace that most people weren't watching the Challenger launch until after it exploded. We were told we'd have a space station by 1991, no problem. The US and Japan were feeding off each other's rampant technological growth and it would never end. The government went from fiscal responsibility to near bankruptcy.
I remember constantly feeling that all rational thought had died and been replaced by gut reactions and responses based on religion or ideology. (Has this changed, or have I just become more used to it?) Religious movements flourished and everyone expected the world to end in a nuclear holocaust tomorrow. You can see it in the way houses from the 80s are already falling apart: people back then did not expect the world as we know it to still exist in 2001. (R.E.M. summed it up in 1987.)
The image that represents the end of the 80s to me is when, on the night of 9 Nov 1989, the TV in my bedroom stopped showing partying Germans for a few moments to jump to the White House to get President Bush's reaction. He actually didn't know what had happened, so he was being told while the cameras were on him, and his reaction summed up the 80s perfectly to me: confusion! He wasn't happy, he was confused, and possibly a bit troubled! It was one of those moments where the mask is pulled off and you see the true meaning of everything, and his face said it all: "We didn't mean for this to happen! This isn't a war we were supposed to win! It was just a distraction!"
I feel like a person from 1980 who jumped in a time machine and arrived in 1991 would be completely lost and without bearings -- that much had changed. But someone who jumped from 1990 to 2001 would probably only need to adjust to minor cultural shifts in entertainment. Except -- my friends who grew up in the 90s are probably right: the internet which is now so ever-present that most people don't bother to mention it was barely even heard of by most people. And so maybe that change alone is dramatic enough to mark a decade.