Whilst watching Dr Who with daughter #2, there was a scene where they couldn't understand some alien speech because the Tardis wasn't working or Dr Who was asleep or something. That started me thinking about all the incompatibilities that would exist between aliens and humans but which are usually overlooked in Sci Fi, apart from the obvious one that they are fairly unlikely to speak English.
We can assume that, if we ever make contact with intelligent aliens, they may well be similar to us in many ways - such as needing oxygen, being constructed from carbon-based molecules, drinking water and being land animals - as there are probably sound physical and chemical reasons for life working like that e.g. oxygen combines so readily with other elements; there are so many carbon-based molecules; the anomalous contraction of water favours life; animals in the sea evolve stream-lined bodies so don't develop limbs with which to manipulate their environment.
But physical things are bound to be different: gravity, air pressure, humidity, temperature, and the combination of gases in their atmosphere; we could imagine an alien struggling to stand up in our gravity or having its head ballooning because atmospheric pressure is too low. Rather than looking suspiciously like a man in a costume, it would probably be of completely different proportions, taller, wider, shorter, thinner - in the same way that other animals on Earth look very different to people - so you're unlikely to be able to look them in the eye without peering up or stooping down (their eyes, if they have them, would be picking up a different range of the electromagnetic spectrum - would it even overlap?).
It seems unlikely that the sounds uttered by aliens would be anything like the sounds we make in speech, anymore than dogs and cats sound the same; the sounds they utter may not even be within our hearing range. So it's unlikely that we would hear them say 'ET' in however croaky a voice.
If an alien turned up on Earth, walking around, shaking hands, hanging out in bars, etc. the scariest risk to both humans and aliens would be their lack of immunity to terrestrial bacteria or viruses and our lack of immunity to alien germs; as H G Wells realised in War of the Worlds over a hundred years ago. New diseases have caused devestation in human population e.g. the Black Death in Europe or smallpox for native societies in the Americas, Pacific and Australasia.
Forgetting about aliens; if humans ever colonised planets around distant stars, however close to the speed of light we could travel to get there, centuries would still elapse for the people back home, essentially making these colonies isolated communities with their own patterns of disease and immunity. A visitor from mother Earth to such a colony could well wreak as much havoc as European explorers did after Columbus.
If you bothered to read on to this bit then well-done; I stopped reading it myself after the second paragraph.