To summarize Richard Neustadt "the president is everything, because that is what we want him/her to be."
To expand upon this point, a lot of things have been changing in federal government over the last 125 years. First and foremost has been the weakening of party cohesion in congress. Woodrow Wilson wrote
Congressional Government in the 1880's and note that, at the time, the Speaker of the House was the most powerful person on capitol hill. During Wilson's own presidency, Wilson took a more active role of leadership with regards to WW1, Panama Canal, Antitrust suits, Federal Reserve, and even his famous 14 points. All of these suggestions were unheard of during a time when solutions typically came from congress, not the president.
Likewise, the power of the president continued to expand over time, FDR's New Deal and the Supreme Court's upholding of such policies, Eisenhower's interstate highway program, LBJ and the Civil Rights act, Nixon nixing the gold standard & creating the EPA, Reagan's policy of pushing for tax cuts; all presidents since have had their power expanded. At the same time, Congress has become more fragmented. The number of Subcommitees has been rising, representatives are less concerned with political unity than with popularity in their own districts, and interest groups have exploded over the last fifty years. Likewise, Congress, not wanting to deal with specific issues, are likely to create government committees and hand them over to the President to run. During Carter's time there were about 700 cabinet and sub-cabinet members appointed by the president. That number has surely risen. Each of these cabinets represents a different power the Congress has entrusted the president with.
I would also argue that the president is more powerful today than ever because of a lazy electorate. Reagan's famous line "are you better off than you were 4 years ago" has become the main criteria for a number of voters. The president is also in a unique position, being the only person elected to represent the entire country. Because of this, people are more apt to blame the president for bad times. Things like the economy, ineffective policies, taxes, unions, corporate responsibility, even the roads we drive on and education we pay for have all become issues by which we vote for or against the president. Because of this, the executive office has had to assume more power to meet our expectations.
All this incoherence aside, another explanation of the president I am fond of (also from Neustadt) is the "presidential power is the power to persuade." As I've mentioned earlier, we are living in a time of a fragmented Congress and a plethora of special interests groups. Likewise, all those agencies I mentioned earlier are also leveraging for more control and power. The president being so connected to so many groups is thus in a unique position to bargain with the individual groups and interest in such a way as to move forward. Likewise, we often judge a president by how well they negotiate their interests and how effective the polices that come from such negotiations are. With the rise of interest groups and political agencies, it's not hard to see how this explanation of presidential powers can mesh with the aforementioned explanation of presidential powers (more government demands amount to more agencies, amounting more more presidential bargaining).
Sorry for the rant, American politics is somewhat an interest of mine. Finally, Ozma (and others who may be interested), you seem like a politico. If you're interested in this, or if you have free time, might I suggest reading Neustadt's book "Presidential Power." It's often cited as one of the most influential books on the American presidency. I will admit it has some short comings, the 2nd half of the book is rather ad hoc and he does display some liberal bias. That said, I do feel it (most) adequately explains the office and expectations of the president.
I hope this helps, I also hope this is the least bit coherent as it's 6AM and I'm a quite tired. If not, at least I boosted my post count by one
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