Part of the reason why Thoreau is vitally important today is because he represents how the spirit of dissent is something that is an intrinsic component to American History. In a more globalized world, where ideas and identities are being changed and challenged every day through the proliferation of information technology and communication, it is essential that we are able to examine individuals in our own past that were able to use their voice in order to speak dissenting ideas in a world of growing affirmation. Thoreau is one of those voices. In his writings, Thoreau emphasized that part of a proper use of freedom is one that does not blindly accept what society does or what government practices. Rather, in a world of growing freedom, we, as individuals, must be able to articulate what we believe as right, stand for those ideas, and stay true to our own moral and political compass as we seek to minimize the suffering of others in our own visions of political and personal self. Thoreau's stand of civil disobedience and activism helped to inspire many in American History who fought for the rights of the dispossessed and the oppressed. This legacy cannot be a bad thing and has to represent a fundamental rationale behind why Thoreau is still important today.
There are lots of reasons, but I'll suggest the one that I think is more important for today. Thoreau was a minimalist, a person who believed that we could be happier with less than we can with more "SIMPLIFY! SIMPLIFY!" This is quite contrary to the way we think today where what we have often seems to be a substitute for what we are.
For what I understand, much of the problem we have today is due to credit cards, or unwillingness to delay gratification and to live within our means. I am old enough to remember life before credit cards, when the best you could do was "lay away,' where you essentially had to pay as you go. Today we have built a false prosperity based on consumption we cannot afford. Worse than that, we are being told that they way "out" of our current economic problem is --- more spending we can't afford. I don't know who this makes sense to, but I'm sure it would baffle Thoreau --- I know it baffles me.
And this is without mentioning Thoreau's emphasis on individuality ... :)
He's also important for the lessons he taught us that are still in use today, not the least of which is the idea of civil disobedience - that it is morally and ethically important for us to challenge unjust law and authority, and to do so non-violently.
This idea has been borrowed countless times over the decades by such notables as Gandhi, when he used it against the British Empire in India, to Rosa Parks and Martin Luther King Jr. in their tactics against racism and segregation. While Thoreau wasn't necessarily the first person to ever think of civil disobedience, he was one of the few to publish about it as a topic on its own, and we certainly use the idea and the tactic today in a spirit I think he would enjoy.
Thoreau is one of our nation's fathers of transcendentalism, the literary movement and philosophical stance that reflection on the soul is important. Taking time to think, not being swayed by conformity and finding one's self are all important ideas Thoreau encouraged through his writing.
There was no real signs of American literature quite yet. In the mid-1850s many literary types were longing for a type of American Renaissance. These guys got together and sort of tried to force one. What resulted was many transcendental writings.
Thoreau still inspires me because I get so caught up in the routine of my work. He reminds me to slow down and reflect on things that really matter, things that build my character rather than destroy it.
He longed to suck all the marrow out of life. You will enjoy Walden as you get started reading it.