I want to know if I can email back and forth to okinawa, japan from
the usa?  How difficult is it?  Does the time difference play a
factor?  Is it common for people to communicate via email between
okinawa and the usa?  Not business but personal emails between
friends.  What are some of the reasons that it would not be possible
to communicate via email from okinawa.  I have not heard from a friend
who is currently in okinawa and wondered if maybe he is not able to
email me.

There is no reason, assuming both party's have access to the internet,
that email to/from Okinawa would be any different than emailing your
neighbour.

http://computer.howstuffworks.com/email.htm