Limbs and the ability to sense its surrounding environment are obvious certainties. For a species to become intelligent, it requires the ability to form complex groups, for which it needs the ability to communicate. Most animals on Earth communicate through a combination or sound, gesture and smell. Gesture alone is not a very good tool for communication, but is a brilliant addition and excels at conveying extra meaning and emotion to mainstream communication. A possibility for alien communication, apart from sound and smell, is touch. This would be associated with a more peaceful society, as simply asking for a coffee involves touching a complete stranger. Sound and touch are obviously the easiest ways for Humans to communicate with an alien species, smell would be difficult, and would require machines to help us detect pheromones, visual communication, such as bioluminescence, like that of the glow worm or fireflies, is an obvious communication tool, that humans could replicate reasonably easily with something as simple as a torch. Visual communication in the non-visible spectrum, such as infra-red, is also possible.
So limbs, senses and communication, what else must an alien species have? A way to feed of course! Again, this is reasonably easy to guess, they'll either have mouth like appendages such as us, including a digestive system, create their own food like plants, or have simple cross membrane absorption (Marine based lifeforms, and unlikely to be advanced). Carnivorous, Herbivorous, Omnivorous, Geophagy (eats stone) or Aerevorous (Gas eating, probably by simple cross membrane absorption, so unlikely to be advanced) are all the possible diets. The most obvious of course is Omnivorous, eating both plants and meat allows a variety of vitamins and minerals needed by species. The most advanced animals will probably be Carnivorous, Omnivorous or Herbivorous, particularly Carnivorous or Omnivorous rather than Herbivorous. But what does this mean? Well we can now put together what a basic alien lifeform would resemble, based on what it eats.
Carnivore: Will have limbs and probably sharpened appendages, will be aggressive and more likely has behaviour similar to a warlike tribal culture than democratic Western societies. The lack of developed intellectual freedom should not be mistaken for being unadvanced, clashes between tribal groups would lead to the development of weaponry, although they would probably lack W.M.D.s, for the obvious reason of eating their slain opponents. Possibly aggressive toward Humans, certainly more inclined than other diets to be aggressive. Most likely sound communication, possibly by smell also.
Herbivore: Certainly more docile than a Carnivorous society, but as forementioned, unlikely to develop intelligence, meat was a crucial piece of early primate development. Limbs are likely, as well as armour, although debatable, the species will have had armour in the past to fend off carnivores, as it became intelligent, it would have lost or downgraded such appendages. Probably larger than any of the other diets, both from (assumed) plentiful food and to fend of predators. Most likely a herd based society, although it's difficult to tell how different herds will interact. Probably won't be aggressive toward Humans, although may be less inclined for active communication or interspecies relations. Sound, touch, small, anyway to communicate really.
Omnivore: Difficult to gauge in terms of docility/hostility, probably much more like us, defensive, but not overly so. Will certainly have limbs, possibly weapon like appendages or armour, although Humans have neither. Will be of average size, like us due to the scarcity of certain types of food versus the wide availability of others. Probably has a similar social system of family based groups, although unlike Herbivores or Carnivores, it's much more difficult to judge. Depends on social system and/or leadership on how they'll react to Humans. Sound is most likely, possibly smell.
Geophagy: Certainly the hardest to judge, as the only solely Geophagist life on Earth are bacteria. Not likely to be advanced, as this diet lacks many vitamins, on the off chance a Geophagist species did become intelligent, it would probably use sonar and be cave dwelling, be quite small and live either in large colonies, like bats, or alone.
Aerevorous: Even less likely to develop into an advanced lifeform than Geophagists. Will lack complex (if any) societies. Probably will not be actively responsive toward humans. Even if a planet had an atmosphere full of nutrient rich gases, the need to have any complex way of feeding is non-existent, an individual can simply stand in one place and breath (Like a plant).
So there we are, basic, animalian-like lifeforms. I have not included any intelligent plant or geological based life because the likelihood of it developing is so low. Gas based lifeforms should be considered impossible, although, a pre-developed lifeform may deliberately choose to develop technology to separate its consciousness from its body for reasons unknown. We humans have already proved we can develop into an intelligent species, so a very biased opinion would be that most other life forms would develop like us. Actually, a carnivorous, aggressive, warrior-like society is more likely to become intelligent, as the need for a technological edge on other tribes becomes apparent. In truth, it does not matter what type of alien we come across, assuming no terrible catastrophe befalls the human race, we will one day meet, or even become, an alien civilization, and we must be prepared; militarily, culturally, politically, scientifically, socially and above all, mentally.
