I think the fact that an inspiring African-American became our president, and was elected so by the people of the United States, speaks volumes about our country's ability to combat racism only a century and a half (or so) after the institution of slavery was abolished. Although I wish I could say racism has been abolished (because it has not), we are at least on the right track to negating its horrid effects in our country. Regardless of how one thinks of him personally or even politically, his ability to inspire many people (including incredible inspiration for people of color) and provide hope for the future is something to be admired.
Aside from the impact of opening a new Pandora's box by the extraordinary and astounding lengths to which opposition groups went to discredit a presidential candidate during--and then a President after--the election, the greatest impact of Obama's election is that a large anti-prejudice and anti-oppression sector of the nation rose up and let itself be seen, known, heard from and counted. One hopes a new sort of "cold war" may emerge in which this impact is escalated and expanded so that a new era of human equality may be established established between America's very unequal shores (one secretly despairs though as long-term politics and partisanship overshadows the great historic event of Obama's election).
I attended the inauguration of Barack Obama in January of 2009, because I knew it was a historic moment and I wanted to witness it. What I saw there was the most diverse crowd I had ever seen. As I rode the Metro into DC from the Largo, Maryland stop (and I have been to DC many, many times), I saw on the faces of African-Americans, a pride unlike any other. On that day, for perhaps the first time ever, they were not going into Washington DC to work or to serve, they were going into the capital to own it. To participate in it. I think that is one very significant aspect to Obama's election is the fact that many African-Americans finally felt a sense of ownership in this country.
I could go on and on about what this historian witnessed that day, but I can sum it up best by something that happened the day after, when I was riding an escalator down to the metro, and I turned to a young African-American woman behind me and said, "Quite a day yesterday, wasn't it?" She smiled and looked me right in the eye and said, "Yes, it most certainly was".
I would argue that the main impact of Obama's election as president has been the increasing popularity of the Tea Party movement.
After Obama was elected, he pushed for a number of things that can be seen as excessive government intervention. He pushed for the "Obamacare" reforms. He pushed for various stimulus packages that have pushed up the deficit. In addition, Obama has not been as aggressively "patriotic" as many people think that he ought to have been. Therefore, the Tea Party has sprung up, largely in response to the policies that Obama has pursued and the things he has said. Of course, if the Tea Party fizzles as a movement, this answer will be completely incorrect in the long term.
I think that you will get different answers to this question, also. I cannot help but feel that the issue of race and ethnicity is extremely important to the Presidency of Barack Obama. The fact that in the 200 year plus history of the nation that a person of color has never served as President, and that the first person of color to serve as President is African- American speaks on its own merits. However, what is even more significant about the Presidency is that its narrative is that of the relative outsider. Obama's background of being born into a middle class family, a separated family, where differences of racial and social identity presented itself at the earliest possible moment in his life makes his narrative different than his predecessors, but more akin to the story of millions of people in America. No better was this seen in his "Forming a More Perfect Union" speech, a moment where a political candidate transcended politics and entered into the pantheon of political thinkers. I think that all of these factor into his election as President.
Another interesting element that presented itself in the election of the President is the very idea that young people can have a profound voice in the political spectrum of America. At a time when youthful cynicism was high, Obama's election represented the fusion of youth. In volunteers who across the nation were young in their passion of the candidate, to the successful appropriation of social networking technology to reach more voters, the convergence of youth, zeal, and technology also helped to make his election significant. It was interesting to note that the selection of Biden as a running mate was something texted to supporters, when in elections past, there used to be a grandstanding political photo opportunity. I think that this element also helped to define the significance of candidate Obama's ascension to the Presidency.