Here's my thesis: the US public alternates between a desire for a "strong" president and a "weak" one.
By strong and weak I mean the assessment that people make of the person's personality type. One is a take-charge type and the other is more of a consensus builder. Whether their presidency can be considered strong is influenced by other factors such as the makeup of congress.
Let's look at the record starting with FDR:
FDR - strong
Truman - strong but accidental president
Eisenhower - weak
Kennedy - neutral to weak
LBJ - strong but accidental president
Nixon - strong
Ford - weak but accidental president
Carter - weak
Reagan - strong
Bush I - weak
Clinton - neutral leaning strong
Bush II - strong
During the periods of weak presidents the country is in the mood to catch its breath after the dislocations that happened under a strong president. In the present campaign Obama is presenting himself along the weak model, by emphasizing bi-partisanship, while McCain is offering the strong model, best illustrated by his continual emphasis on militarism. Hillary Clinton is seen as a strong type as well, partly because she doesn't discuss the same bi-partisan themes that Obama does, but also because she must overcome the inherent image that a woman must be weak. She also carries some of the aura of Bill with many people considering the two of them as alike and imputing Bill's personality onto her.
If my conjecture is correct then Obama should be the nominee and probably the president. He best captures the zeitgeist of the moment. The issue of prejudice makes the actual outcome of these races uncertain, since we have never had two candidates who represent groups which have never achieved high office running at the same time before. I'm not prepared to guess whether zeitgeist or prejudice will by the most important factor.
Having a weak president doesn't necessarily mean that this person's administration will accomplish little. Eisenhower had a very aggressive foreign policy, he just left it to John Foster Dulles and others to run. On the other hand I think Carter's appeal was that he would be more of a caretaker and give the country a chance to catch its breath after the prior period of upheaval.
Obama supporters seem to want a person who promotes the weak model, but are also interested in "change". Does this promise of change mean a change in the style of governance or a change in government policies? It's not clear. Most of Obama's policies are similar to Clinton's, despite the attempts of both camps to emphasize minor differences. I don't see strong public pressure to resolve outstanding issues, with the exception of the war in Iraq. Health care, poverty, economic stagnation and inflation are all of concern, but are not bringing people out into the streets as happened during the Vietnam war.
As I said this is only a theory and not an attempt to explain why one candidate should be preferred over the other, so please don't tell me why X is the best person for the job. I'm sure you are correct, whatever side you are on.