1. Aliens won't look humanoid
2. They won't speak English
3. They will be more intelligent than us
4. They won't share our value structure
5. They won't wish they were human
Most comments agreed, though a couple of people took issue with number 3. Anyway, here's summary of what people said.
1. Most agreed that, different to most films/games/TV series, the non-humanoid alien was best for books and stories. There was a big discussion as to why other media still go for humanoid aliens, despite the power of computer graphics; in brief, films etc. have to gain empathy from the viewer much quicker than a book does, and so humanoid central characters, or at least anthropomorphic (essentially humans even if in disguise) characteristics are the norm.
2. Avoid the 'humans are special' trope (my rule 5 above), which one person pointed out was the same as the 'Everyone wants to be American' trope.
3. Those that accepted my 'more intelligent' rule thought it should be intelligence at a completely different level. Somebody pointed out that genetically there's not that much difference at DNA level between us and chimps, but the actual differences in what we achieve are enormous.
4. Vastly more intelligent aliens would probably not be interested in anything we had to say. This 'indifference' can be used to great effect in books, creating mystery and 'alien-ness'. Incidentally, even less intelligent beings can be indifferent to great effect, as with the Bikura in Dan Simmons Hyperion.
5. Intelligence is a product of environmental need. If a species is surviving nicely, thank you very much, and has a stable niche (like the cockroach, or jellyfish), then there may be no need to evolve, and intelligence level will plateau. This is a challenge for the SF writer, to indicate why intelligence might increase, perhaps due to interaction with other species (e.g. David Brin's Uplift saga). Other options are eugenics, cybernetic augmentation and psychosurgery, and inter-species mating.
6. Some stated that intelligence would be limited by a species' physiology, with a bit of leeway for social evolution. Similarly with civilization - it would go so far and then just add more of the same, despite technological advances, the end result being social advancement and/or stagnation.
7. The most convincing aliens have an entire culture behind them, which takes a lot of time and skill to develop.
8. Further to (7), space-faring aliens will likely have met other aliens, and so there will be cultural mixing or at least references between different alien cultures. This is akin to normal fiction, where writers mention a character's half-brother or uncle or day-job, to give the reader the feeling the character has a life off-page. This suggests a galactic society in some cases, and a long history to boot.
9. Intelligence can be electronic, as in the 'Minds' of Iain Banks, or in some of Alistair Reynolds' works, or the later Dune sequels by Brian Herbert and Kevin Anderson.
10. Advanced cultures might be less technologically-focused (i.e. been there, done that), and be more focused internally (zen aliens). This might mean they don't need ships to travel, and maybe are seeking some kind of (Stargate-like) ascendance to a higher realm, or just trying to get out of this galaxy/universe (as in Stephen Baxter's Xeelee aliens). In brief, they might be more concerned with 'inner space' than outer space.
Since posting the blog I've thought of two other facets which can be useful for 'building' alien cultures:
11. Mode of reproduction. A lot of human social life is actually geared around reproduction, and in fact it is probably the biggest driving force in nature. The actual mode of reproduction will affect social behavior and traits which can be quite alien to us. Incidentally, someone pointed out that insectoid aliens (as brilliantly done in the Ender series by Orson Scott Card) have been a bit overdone, so new approaches would be preferred.
12. Perception. Humans are generally very visual, and a little bit auditory. What if aliens weren't, or were perceptive of a different or vaster range on the electromagnetic spectrum, or telepathic, or just thought much faster or slower? Perception shapes a lot of who we are, ditto for aliens.
So, where does this lead me? I'm writing book three of the Eden Trilogy, and have some work to do as I already have several species far more intelligent than humans. For me the biggest lesson is that aliens, even if they have some interest in humanity, must never, ever be 'human-centric'. Aliens, as the dictionary says, are different.
The Eden Paradox available in paperback and ebook on Amazon
Eden's Trial available in ebook on Amazon
Eden's Revenge - available only in my head ... until September 2012.