Humanoid aliens
There are lots of science fiction shows and films in which alien species are basically just like humans with cosmetic differences - different skin types, strange ridges on their heads, and so on. The classic example in Star Trek, but it also happens in Babylon 5, Farscape, Blake's 7 and many more. Even Star Wars has this problem; its aliens have more variation in physical size, but still the majority of species have an overall humanoid shape.

IT'S REALLY DULL

Can any writer of science fiction shows really believe that this makes a good story? Out of all the vast number of exciting possibilities of what aliens could be like, we are handed aliens that are so like humans that there's effectively no difference. This is really really boring.

First contact between two species should be an exciting prospect filled with mystery. What will they be like? Will we be able to communicate with them? Will they see reality in essentially the same terms that we do, or will their world-view but radically different? Programmes like Trek take away all this excitement; when the crew of the Enterprise meets a new alien race, it's just like meeting members of another human culture. This takes a lot of the excitement out of space exploration, because when pioneers go out into the galaxy to look for life, they find other species which are basically just like themselves. Yawn.

IT MAKES NO SENSE

Why should aliens all be roughly human-looking? If they all eveloved separately on different worlds, their courses of evolution would have taken vastly different paths. Even if they evolved on planets with similar ecosystems, there is no reason why evolution should have followed a similar course. Ceratinly the internal biology of the cell would have developed in completely different ways on different life-bearing planets.

It's possible that the early course of animal evolution might follow a vaguely similar path in the majority of cases: individual cells form into multicellular colonies, the colonies which are most efficient at preying on other cells are those that form bag-like structures, and these bags subsequently develop specialised tissue types with gut cells on the inside and muscle cells on the outside, thus giving rise to primitive worms. But from the level of the primitive worm upward, evolution would have so many possible paths open to it that there would be no constraint to move in one particular direction. And this would only be physiology at the level larger than a single cell that would be similar between different planets; as I said before, the internal biology of the cell would be very different in each case.

Races from different worlds should certainly not be able to interbreed, and a drug which has a physiological effect on one race should not have the same effect on another.

SO WHY DO THEY DO IT?

Why do the writers of science fiction shows continue to churn out the same old thing? There are several reasons.

They're lazy. It takes a lot less thought and effort to write a script about humans dealing with essentially human-like creatures than about humans dealing with totally alien creatures.

The public laps it up. Programmes like Trek have a huge following. People don't seem to have a problem with this crap. They just accept it. I must admit that I enjoy Trek despite the fact it's so stupid. I enjoy stories about Klingons and Ferengi even though I know they make absolutely no sense.

It's cheap. Hiring actors, putting some wierd stuff on their faces and putting them in front of the camera is a hell of a lot cheaper than the special effects needed to make believable-looking non-human aliens.

You can write human drama. People want to see stories about characters they can understand doing things with each other. The stories that are told in these programmes are essentially human stories, and so they appeal to people. The average viewer wouldn't be so intereseted in stories about totally alien things.

'It's no fun kissing a slug.' - Gene Roddenberry. The fact is, everyone wants to see alien-human sex. It sells TV. There is some sort of fascination about tapping off with an alien which has been with us since the beginning of SF, and before that came across in stories of men and mermaids, or gods taking the form of animals to impregnate young women.

You can make convenient 45 minute episodes. Meeting some prvieously unknown race of human-like creatures and doing stuff with them can be packaged neatly into an episode of a TV programme. You can't do the same so simply with making first contact with completely alien things that you can't understand.