The original comparison was drawn from how well both leaders used their charisma to gain national power. Both Hitler and Obama were practically nobodies who burst onto their national stages based primarily on their attractive personalities - their ability to command crowds and use persuasive/emotive oratory.
A genuine parallel can be drawn between Hitler's rise to power and Obama's election, particularly if you analyze both leaders' respective party. Both Hitler and Obama, prior to becoming heads of state, transcended their political party (Hitler in 1933(?) and Obama in 2008). By "transcended", I mean that both leaders essentially "became" their respective political parties. Hitler's personality cult successfully turned the Nazi party into the Hitler party (metaphorically speaking). The policies of the Nazi party were dictated solely by the whims of Hitler. And if Hitler were to be incapacitated or killed, then the Nazi Party would literally have been shattered.
Likewise, the 2008 election was not really an election between Obama and McCain. If you were voting, you were voting either for Obama or against Obama. Pundits were either coming up with reasons to vote for Obama or reasons not to vote for him. The media coverage of McCain was abysmal. During that electoral season, the nation's attention was turned toward Obama almost exclusively. For that short period of time, Obama was the Democratic Party, and carried its fate with him.
Of course, after Hitler and Obama gained power, they took radically different paths. Hitler relied on authoritarian repression, propaganda, and his strong personality cult to maintain hegemony. Obama never had that kind of personality cult; he relied on his natural political skills. He can't overcome the inherent institutional divides in the American system of government with simple charisma (constitutional limits, inter/intra-party rivalries, etc.). Also, it's important to note that Obama is a much more polarizing figure than Hitler was in 1933. Hitler had a strong national backing that Obama doesn't currently have.