Americans are pretty smart except when they vote for Republicans.
Americans are pretty smart except that they have a lousy health care system.
Americans are pretty smart except our education system is going down hill.
Americans are pretty smart except our infrastructure is falling apart.
Americans are pretty smart except they waste time and resources on "reality TV" in order to escape reality.
Americans are pretty smart except they allow their media outlets to function as propaganda organs.
Americans are pretty smart except that they're NOT!

That's the only except-ionalism I can think of.

That somehow our economic and social history are developing outside of the streams of other countries and we will have a different fate even when our trajectory is similar.

A decent place to start.

http://en.wikipedia.org/wiki/American_exceptionalism

The belief that America is unique in that we are the only nation with indoor plumbing, voting, a constitution, civil rights, electricity, hospitals, religion and human decency.

I can't even ride with "Constitution" and hospitals is even unclear.

to find the definition. Otherwise, you're going to wind up with a wiki source or some cynical definition.

at first the continent through Manifest Destiny and later the globe through false notions of things like an American Century. The belief that we have a duty to "civilize", democratize and teach capitalism to tame savage brown people (and kill them when necessary).

The belief that we, as Americans are exceptions to the rules of history, empire, geopolitics, environmental disasters and sometimes karma.

xenophobia. We have a right to loathe, dehumanize and discriminate against anyone who is not like us, because we are this exceptional people, better than anyone else in the world.

That there's some je-ne-sais-quoi about the United States that exists only in the US. What the attributes in question are depends on the person claiming them, but they all boil down to claiming some unique status, sometimes with hints that it's divinely bestowed, sometimes with the idea that the American people are the only ones who Get It enough to bring said status about. As someone else said, the implication that the US is exempt - excepted, even - from various "historical rules" is a big chunk of it as well. (Fukuyama claiming history was over when the US "won" the Cold War is a pretty classic example of that; on a far sillier level, people refusing to go metric out of a sense of patriotism is another.)

Most people I know who believe in it claim it applies to all Americans, but most of the people I know claim that also have a "well, real Americans, anyway" clause that excludes people they dislike.

It's a pretty old attitude; it hugely predates the Cold War, for certain. Most ideas of that kind of vintage can't really be nailed down to "this guy came up with it on this date" precision, since it's usually a stream of other ideas all merging into the resultant one.

Speaking from a perspective outside the US, there's an attitude of bafflement that the rest of us don't automatically want to emulate what's going on in the US in every way, shape and form. Canada in particular tangles with that a lot, especially as our current government is, for all practical purposes, the Republican Party, but a lot of us out there see a kind of confusion about how everyone else reacts so negatively to exceptionalism.

Plenty of other countries have their own equivalent, too; most of them just have slightly less raw power at their disposal, so it's a little more difficult to really get into the "why doesn't everyone want to be like me?" confusion.

In other words, a way to prevent the United States from ever becoming a "more perfect union."

(Hey! When did we get spell check back?)