There are a number of characteristics that can define the difference,
but
the lines are frequently blurred.

Sean, I'm surprised you didn't mention sidecut! Again, not a hard and
fast rule, but freestyle boards often have a tighter sidecut than their
freeride brethren. (Sidecuts are usually expressed as a radius -
smaller number = tighter sidecut). The tighter sidecut allows one to
make tighter *carved* turns... which is helpful for getting more hits
in the halfpipe. Freeride boards with mellower (larger radius)
sidecuts are better at making larger, faster carved turns... this tends
to increase stability at speed. Suppose you have two boards of length
X. The board with the tighter sidecut will "feel shorter" in a sense
since it makes tight turns with less effort. The lines here are
frequently blurred as well... look at Burton's specs... their sidecuts
are all in a very small range. In fact I personally think they are
making a mistake by not offering anything with a mellow, surfy sidecut
this year!