Usage would be something like "Fred was a burger flipper
at McDonald's, a regular Joe who thought he had nothing to
offer the world" or "Bill Gates was just a regular Joe until he
launched Microsoft, a move that would change his life
forever." Usually the term "average Joe" is used in reference
to one person in particular, to explain that he is nothing
special -- but you wouldn't often hear of an anonymous
person described as an average Joe. For example, I've never
heard "I bumped into an average Joe while walking down
the sidewalk." There's usually a specific person in mind
when using the phrase.

Also, the term is always male. I don't think women can be
average Joes, but the name Jane may be substituted for
similar effect (though less common).

> Used this way, joe and guy are not proper nouns.

At first I was going to disagree and say that Joe is a
person's name and thus should be capitalized regardless of
this usage. However, I then remembered that Guy is also a
person's name, but the word "guys" is so common now as
an improper noun that I suppose "joe" could be also used in
the same way. I just capitalize it out of habit, and because
this usage is not yet as common as the word "guy," which
has been around for pretty much forever. Case in point, Joe
is still a very common first name, but one does not
encounter people named Guy very often anymore. The
lowercased term "guy," casually meaning "man," has made
its way into the vernacular, as I think "Joe" is in the process
of doing -- but it's not completely there, yet.

Additionally, I have heard the reciprocal "Joe Average" used
as a random full name when describing nonexistant people.
Sort of like "John Doe" or "Joe Schmo." This would
commonly be used when creating a theoretical situation
with which to make general assumptions. For example, in
my microeconomics class, my professor said something
like "Suppose Joe Average goes to Wal-Mart and buys a
dozen boxes of cereal." We know that Joe Average is not a
real person, but he is used to put a name on a fictional
everyman character for the sake of constructing an
example. Very colloquial, I should add.

-useless"just two cents from the man on the street"ness