Alien-ness (Alienity?)

I'm not sure what the proper term is, for "the quality of being alien". But I was reading the back of a novel I bought recently, and I got to thinking of what it is that makes it so hard to write about truly alien beings.

Footfall is basically the story of a war between modern-day earth and an invading race of giant tribal space elephants. That ultra-condensed description makes it sound corny, but the more you read, the better it starts to sound. Larry Niven is a genius of a writer, and once you get over the premise, everything in the story is clever and logical and completely realistic.

But did you notice how easy it was to describe the aliens? They're basically the combination of an old-fashioned societal structure (nomadic tribes), an antique world-view (if it's weaker than you, eat it. if it's stronger than you, beg it not to eat you), an animal (elephants), and, to keep it interesting, let's say they're twice as big as people, and they're from outer space.

Okay, okay, I over-simplify a bit, but that's the rough of it. Nothing in them is truly alien.

So, my question is, what are the most basic, most fundamental traits that we can't seem to discard when we try to imagine aliens? I'm not the first one to ask this, and we have come a long way from the times when aliens were just odd colored people, or were distinguished by the shape of their forehead ridges. But we haven't fully answered the question yet.

The first one that came to me was that of reproduction through binary sex. One gender and another gender get together and make a new person which is a collage of the two originals.

What would an alien that defied that be like? Perhaps they start off smaller than us, and have trilateral symmetry, like pyramids with a clearly marked "this face forward" (their rear leg is much meatier than the front, sorta like Niven's own Puppeteers). And at the apex, two meaty appendages point in opposite directions, perpindicular to the forward face The creature continues to grow larger for its entire life, and as it gets larger, it becomes slower and more ponderous. Its memories are distributed at random in actual physical locations throughout its body. Its life "ends" when it suffers some sort of physical trauma to a spot along the increasingly sensitive and tender centerline of its body (which vertically bisects its face). At this point, the creature falls apart as if cut with a sword, and two new creatures are born. It should also be noted that it has a vivid, near-permanent memory of everything that it learns, but is actually pretty slow to learn "new" things. Each new creature gets half of its "parent's" skills, memories, and interests. Only very rarely is a skill or memory cut in half and shared between "brothers".

So we don't have traditional heredity, but that's okay. Learning can, to a great extent, take the place of what our earthly paradigm accomplishes through genetic drift and survival of the fittest. The success or failure of a tribe or individual depends on the content and number of useful skills or memories they possess. There is no such thing as childhood, as a creature is "born" with a robust enough set of skills to let it thrive. At birth, they are also gifted with increased vigor and healing abilities. And smaller creatures trigger a patience/tolerance reflex in larger creatures, so that they tend to be treated indulgently until they become a respectable size.

That could work, right? I bet somebody could write a really interesting story in that world, assuming the navigated the tricky "this is what makes these aliens alien" chapter.

Vernor Vinge does a really virtuoso job of this in A Fire Upon the Deep. His creatures use sexual reproduction, but with a really brilliant twist.

So, here's my other question: Can anyone recommend other authors that are really good at coming up with a premise that's actually, truly alien, while still plausible? Don't say A Mote in God's Eye - I already know. And I've read pretty much everything Niven has written.