Has any SF/F author really done an adequate job in inventing a
realistic alien world (society)?  Of course there are the blatant
substitutions (well, they have 3 sexes!  well, they have this sixth
sense!  well, you start out old and grow young!  well, etc.), and
copies (Western in Space!  Detective story in Space!  Kid and her Pet
in Space!), but who has really invented a setting totally alien to what
we've experienced?  CAN anyone, once they've learned how to pigeonhole
the world enough to be considered an adult?

I had a discussion with someone once about whether anyone could be
truly creative.  Like, could you think of a truly original animal,
rather than a combination of existing ones or at least their
characteristics.  The poor guy spent the next hour trying to think of
something that wasn't a variation on something else, without realizing
that if he thought of it, he'd have to describe it in terms of existing
things (well, it's sort of liek feathers, but different).  I have no
idea what any of this means.

Has this topic been done to death?  PO that it hasn't: I've been told
(though I don't know from real experience) that the Kif in
C.J.Cherryh's Chanur series are about as alien a race as exists in sf.
On the other hand, I think the person who said that was the ANALOG book
reviewer, who I trust very little.  Hmm.  The only way I could think of
to do a totally alien psychology (which to me is far more interesting
than an alien physiology -- for that, try Jack Chalker's Well of Souls
books; there's a race that looks like stripes of color moving through
the "air" (which is non-oxynitrogen based) and are almost too alien to
communicate with) was to make actions apparently random.  ie, this
race's psychology is so alien to ours that we simply can't comprehend
their motives for anything.  This might make a rather dull story
(though it was used effectively in the comic book MOONSHADOW -- the
Gl'Doses are pretty alien).
   One argument might be that any race close enough to us to
communicate with at all must have some similar features.  Another
step might be to say that they'd have to be Carbon-based life, or
humanoid, or evolved under similar conditions (which brings up the
question of what other planets might be like, whether life can even
evolve on non-earthlike planets, what life that evolved in space might
be like, whether there ARE any non-earthlike planets, whether there are
any other planets, period (it's a mighty big cosmos, though), and so
on).  Another sample from comics: an Alan Moore story in SWAMP THING
about machine-based life that lives solely in space.  Of course, in a
lot of ways it was pretty humanesque.
   And of course, there're those who say that alien anything, meaning
anything inconceivable to the human mind, is (by tautology)
inconceivable to the human mind, and that therefore we shouldn't even
bother to try to think of truly alien races/thoughts/communication
modes.  Since that point of view makes speculation pretty pointless, I
try to avoid it even though I think it's a good point.  I thought Piers
Anthony's methods of communication throughout the CLUSTER series were
prety good, and the alien races pretty alien (but only in physical
appearance; definitely not in psychology).
   To attempt to answer one point brought up above: I think it IS
possible to describe something in human terms simply because that's the
only analogy we have, without it actually BEING in human terms (well,
there're these things that are like wings, see, but they're not REALLY
like wings, but that's the closest we can get to describing them).  In
this way, it's possible to come up with things totally unlike things we
already know (how about the computer as an example?  That was a pretty
alien concept: describe it as "well, it can sort of do arithmetic.
Well, no, it's not intelligent, and it's not alive, but it can do
arithmetic anyway.  How?  Well, it's like one of those mechanical
engines.").  My argument has totally derailed by now; please excuse the
long-windedness.  Someone else's turn.