Functionality vs. WOW-factor
Every design is up to the challenge of being unique and original, while, at the same time, being convincing, functional and believable. That's of course pretty tough as by now almost every possible idea has already been depicted somewhere in the vast sea of comic books, novels, computer games or feature films. I could describe it like that:
The cooler something looks (in other words, "wow!"-factor is high), the less functional and/or common sense it often appears. Weird shapes and alien structures will definitely look original but will they be convincing enough? Not to mention that they may be confusing for the potential viewer and that's something that producers/directors are often afraid of. Average movie-goer have no idea about design, so everything on screen should be easily identifiable, within a few seconds, even if it's an alien ship. Just look at 'Avatar' and how easily recognizable all the designs are, so even children know what they're looking at - helicopters are helicopters, only with two propellers instead of one, horses are horses, just more colorful and with two extra legs, aliens look like people only they're blue and bigger, jungle is a jungle just bigger, etc. Even if it would be the first sci-fi movie the audience have ever seen, there would be little confusion.
On the other hand, if you take a look at some really cool designs (especially in games) and take a while to analyze them, you may find out that they are, in fact, very impractical and make no sense at all. It's good to remember that most of the designs are suppose to show things that, theoretically, would be built by someone in the fictional past or fictional/possible future. So now ask yourself a question: "Why would those people (engineers, technicians, blacksmiths, whoever) create the thing that way? Would that thing work well in such or such situation?" For example, modern cars tend to be more and more aerodynamic and sleek. If you design a futuristic car to be used in a big metropolis that's really chunky and blocky, even if it looks really good, think of a good explanation. Why so sudden change in the vehicle design approach? If you design a house with strange looking doors or windows, why do they look like that? There's nothing wrong with rectangular ones as far as we know, so why yours look like movable fish scales? Unless it's suppose to be some custom job, are they really functional? The same goes for fantasy, even though it's usually more forgiving than s-f. Why would an ordinary orc, say, a cannon fodder of a huge army, wear an armor that looks like a few weeks job? He'll be probably slashed and hacked within the first few minutes of a battle, so why would anyone bother with all the carvings, paintings, golden rims, etc. As cool as it may look, it simply doesn't make any sense.
In short, although it's not an easy task, always try to match attractiveness and originality with possible functionality. That's true, that if you design for game industry, your clientele is usually aged between 15-25 and it doesn't concern itself too much with technical details as long as something looks cool enough to grab its attention. On a movie screen, on the other hand, something may show up only for a few seconds and than disappear for good before you have a chance to take a closer look. Still, that's the rule you should keep in mind if you want your designs to look not only attractive but also solid and well thought.