When I was younger, newsmagazines and regular people spent a lot of time on deciding what mood the world was in during a given decade. The practice goes back a while (the Mauve Decade, the Gay ’90s, others), but it mainstreamed hard during the ’60s, and then the ’70s were a reaction to the ’60s so they needed an assessment too, and then the ’80s were a reaction to the ’70s, and the ’90s were a reaction to the ’80s, so the chain kept going for a while.

The assessments had something to do with big-league events in the world, such as assassinations and wars, but their heart always seemed to be this: for the past few years “we” have been behaving and thinking differently than we were a few years before that, and these changes in thought and behavior amount to a whole new climate for life. How shall the climate change next?  
The official sequence of decades went like this:  ’60s (idealism! upheaval! violence! challenges to established norms! liberated lifestyles!), ’70s (cynicism! self-absorption! stagnation! liberated lifestyles!), ’80s (money! traditional norms! consumption!), ’90s (austerity! youthful ennui! spirituali — wait, the economy’s back up — dotcom!).
I make that sound pretty stupid, but the changes in behavior and attitude that happened just before, during, and just after the 1960s really were a sea change. If you want to refer to them all, “the ’60s” is the neatest way to do it. During the 1970s and ’80s, people really were working out what to make of the changes, how far to take them and how far to retreat from them. The silly aspect of the decades business, never a small thing, got larger as the post-’60s consensus worked itself out.
By the ’90s we had a new generational shift, so chances looked good for a new, highly distinct zeitgeist unit. But Gen X didn’t really have much of a new set of attitudes. People thought they were pretty mopey, but that turned around when they started finding jobs. Therefore, the newsmagazine aspect of the ’90s changed almost in mid-year. One month you were hearing about how “the ’90s” were a time of hardship, disaffection, creepy enthusiasm for serial killers, and so on. Then, all of a sudden, “the ’90s” was a period of crazy amounts of money and consumption — “excess,” as journalists like to put it. 
With its raison d’etre running out, the decades business also encountered two important technical obstacles. First, there was a new millennium. That looked like a fat invitation for more zeitgeist assigning, but guessing at a spirit for a whole millennium makes one feel pretty stupid. At the same time, figuring out a decade looks too trivial to bother with. Second, it’s tough to talk about a period of time when there’s no number to go with it. What do you call a decade whose years are marked out by 0, as in ’01, ’02, etc? Somebody in Slate suggested “the Oughts,” which was enough to show the decades business was in trouble.
Finally, 9-11 happened, and then the Iraq invasion, and then Barack Obama’s election. I’m tempted to think that these events have provided us with such immense milestones that the this-decade-vs.-that-decade parlor game has slipped people’s minds. On the other hand, the 1960s and 1970s had their own jumbo events, and what happened was that people assigned the events to serve as markers for the start and end of the decade-as-zeitgeist units — I mean the assertion one used to hear that “the ’60s began in 1963 and ended in 1974,” which is keyed to the Kennedy assassination and the end of the Watergate scandal.
With the 1980s we had a pair of big, zeitgeist-defining events to get the decade started: Lennon shot, Reagan elected, and (as newsmagazines reminded us at the time) the ’80s are underway. 
But no one talks about how our current decade really began on Sept. 11, 2001, or how it ended on Nov. 5, 2008. Back when the market crashed in ’87, Newsweek got out in front with a cover story saying, in so many words, “The ’80s Are Over.” Nothing like that now.
Maybe people are getting smarter, though that’s never a good bet. My own theory is that all this goes back to the baby boomers. The ’60s-’70s-’80s chain of zeitgeist assessments was a means of arguing about where those crazy kids would take the culture. The two choices being discussed always seemed to be idealism/libertinism vs. traditional values/greed. Conservatives would say the choice was between libertinism and traditional values, liberals would say it was idealism versus greed. What it came down to was arguing about whether the country would continue to change in line with the changes kicked off during the 1960s or whether it would swing back to the pre-’60s status quo.
Now the baby boomers are all going to have urinary problems and then die, and it looks like nobody else is planning any big departures from present modes of operation. Which probably means that steam is gathering for a direction that’s so new nobody will figure it out until we’re 10 years into it. At which point we’ll hear about “the Teens” or “the Age of Palin” or some damn thing.