I've responded in a few places, and might as well try to cover many of the points here.
Culturally, "the 60s" was that period of time from Kennedy's Assassination to Nixon's resignation; from the Beatles to the Sex Pistols; from Twister to Dungeons and Dragons. The beginning and end points are fairly easy, though they are not sharp dividing lines. The highlights and lowlights are harder.
One of the major driving forces of the 60s was the Baby Boomers. At the time, it was the largest number of babies born in the US. By a lot. 76 million post-war children, all growing up. The Census Bureau defines the demographic as babies born between 1946 and 1964, though some (including 1961 baby Barack Obama) cut it off in 1960.
1955 is squarely in the middle, the year of Disneyland, McDonalds, Steve Jobs, Bill Gates and me.
By Woodstock in 1969, the first of the Baby Boomers had graduated college and had good jobs, what we would later call Yuppies. Many 40s and early 50s Boomers were in college, away from their parents for the first time, with a flexible schedule (i.e. they could skip classes) and with a moderate amount of disposable income.
The older generation -- Browkaw's Greatest Generation -- had grown up during the Depression and many came of age during WWII. In that period 1963-1969 the older generation's significant events included, Kennedy's Assassination, The Berlin Wall, cars with radios, the rise of tv newscasting, the assassinations of Martin Luthor King and Robert Kennedy, the Vietnam War, the election of Nixon and the Moon Landing.
Those events affected kids differently than adults. For many, including myself, Kennedy's Assassination was our first major memory. Nothing quite as disquieting and shocking happened until 9/11. The kids -- the younger generation -- didn't share the change. They lived it. Tv news wasn't a change in medium, it was the medium. When the Beatles came to America to be on the Ed Sullivan Show in 1964, at last the kids were the main audience. Three months after Kennedy's death, they gave us a much needed morale boost. And mainly, they gave the Boomers, many by then in their teens, something they could call their own.
And that's where Woodstock comes in.
Woodstock was an impetus for the new technological innovation: Stereo. Within a few years, top-40 AM radio was overtaken by album-oriented FM stations. Concerts like Woodstock demonstrated that people wanted more from music than canned three-minute pop announced by djs on speed.
Let me see if I can place this in understandable context. Lots of people remember Woodstock 40 years ago. 40 years before Woodstock... they barely had commercial radio. Commercial radio started in 1920, only in a few places, and not many people had radios. Even so, the early part of the 20th Century saw a tremendous technological boom in audio. But low tech: The amazing thing was that you could hear broadcasts or recordings at all. If you were rich you could hear low-fidelity phongraph recordings. If you were moderately affluent (or knew someone who was) and lucky you were within range of a radio station and had a staticy AM radio. The world changed quickly, because of the new technology; the Grand Ole Opry started broadcasting in 1925. Mass media effected America greatly; the time period is known as The Jazz Age. 40 years before that, in 1889, the world didn't have radio or recorded music of any kind. Your music experience was very likely limited to live performances and/or church. Before broadcast audio, people built opera houses in the middle of jungles. Many who were alive at the time of Woodstock remembered back when they had no more opportunity to hear music than they would have had in Ancient Rome.
Baby Boomers grew up in a world that never lacked radio, tv or recorded music. This is one of the biggest changes in culture ever, far more of a cultural divide than wars or assassinations. The medium is the massage.
Other events affected Boomers, of course, and you didn't have to be a Boomer to be affected by younger generation events. Vietnam was starting to get Boomers killed. The injustice of race relations and the hypocrisy of behaving even worse than our opponents in the Cold War were at odds with the teachings of almost every parent, not to mention the teachings of Jesus (and every religion). Many people born before 1946 were affected as well, of course. Vietnam was the most immediate problem, and it was a problem for everyone.
Politically, the anti-war movement cohered several disparate groups that had overlapping but not identical goals: The environmental movement (which resulted in the Clean Water Act and the EPA within a couple of years), the free speech movement, the anti-war movement (which elected Nixon and his "secret plan to end the war" and eventually saved thousands of lives by getting us out), the drug culture (which, lacking political clout, screwed up royally but at least provided an alternative to the drunken Valium-hazed 50s) and dragged the US kicking and screaming out of the McCarthy-era Cold War hatred and suspicion.
In and of itself, Woodstock did none of that, but did serve as a focal point for many of the issues, a rallying cry for the emerging political and economic power of the Boomers. Within a few years, 18 year olds could vote. Predictably, this didn't change the political landscape in any major way, but did signal a subtle shift in demographics that has been, in general, for the good. Maybe America didn't have the political will to pass the Equal Rights Amendment, but women's rights were on the table and have never gone away. The older generation pointed with pride toward the moon landing, and rightly so. The younger generation had Woodstock, less than a month later. It was a powerful one-two punch: As a nation, we simultaneously thumbed our nose at Communism and anal-retentive pseudo-patriots.
Woodstock was the last gasp of innocence, where half a million people could just show up and police themselves. Compare the security a few months later at Altamont. At Woodstock, despite the rain, no one was electrocuted; the techies did their job. Despite the lack of sanitation facilities, toilets and water, with kids running barefoot through cow pastures, there was no infection reported, nor cholera or related illness; the engineers (especially family friend Ed Silvers who dug the wells and poured chlorine everywhere) did their job. Despite the unrestricted drug use, there was only one reported (but unconfirmed) death from drug overdose (though there were other drug-related problems); Wavy Gravy and his crew did their job The largest rock concert in history was the most disorganized... and the most peaceful.
Woodstock was also one of the last major events without extensive television coverage. A huge number of people just arrived and were left more-or-less alone. The movie covered some ground, but wasn't on tv screens for the 6 O'clock news. Within a few years, almost any event from anywhere in the world had live or taped tv coverage. Television news was not going to miss out on another big story, no sirree.
In retrospect, Woodstock was a prime example of a flash crowd, what we would now call a DoS attack. It had nothing to do with politics, it had to do with getting away from your parents (which I did by going to Disneyland while my parents were at Woodstock). Woodstock was the last gasp of Flower Child innocence. After that, it was Real World (tm) laissez faire capitalism for the underground drugs and slogging in the grassroots for political power.
After flexing its muscle at Woodstock, many entered political power still playing songs from the concert. We cleaned up the water and air, got us out of Vietnam, almost passed the ERA, invented the personal computer and used the internet in ways that hadn't been imagined by its developers. Some put away the beads for a suit jacket and tie, switching from marijuana to cocaine. Some stayed children, away from the world ala Thoreau. But most simply grew up. The world was changing and finally we were part of it.
It's an odometer year. Let us have our fun. If you think the 40th anniversary is waxing nostalgic, wait until the 50th.