There are two types of aliens in science-fiction: the monstrous of mind and body who only wish to destroy, and the incredibly human who are sentient and sly. Rarely do we find a race that is somewhere between. Sentience and intelligence is almost a guarantee of a human-like body, and the lady aliens? They will definitely have boobs.
Why this can be bad: There are several things at work here when we're crafting an alien species. We have a tendency to think ourselves the top of the food chain and therefore the most intelligent species. Anything on par with or superior to us must therefore look similar to ourselves. As species of organisms on earth grow more advanced in neurology, they come to have a very specific pattern: major sense organs all located around the brain and in the head, two arms, two legs, tendency for bipedalism, et cetera. And, of course, there are the influences of pop culture such as earlier seasons of Star Trek, which lacked the budget and technology to create intricate unhuman characters capable of being on screen for more than a couple minutes.
Put this all together and it culminates in many uninspired alien designs. The sentient beings end up closely resembling us, if not being nearly identical. Some of this can amount to laziness, but a lot of it has to do with the factors I listed above. While we can't exactly argue whether or not this is realistic, it nevertheless becomes tiresome when aliens devolve into humans with strange skin colors and maybe a few other "exotic" features. While fantasy can have this issue (dwarves, elves, and humans are all pretty much the same as far as special variation go), they at least tend to exist on the same planet and therefore go through the same evolutionary process. Aliens do not have this excuse.
How you can fix it: To ask you to create a dozen completely unique alien species for your science-fiction novel/game/movie would be insane. It's very hard for us to think of creatures uninspired by ourselves or the world we see, and similarities to humans make it easier for the audience to envision or connect. However, I would challenge you to make your sentient aliens more diverse. Octavia Butler does a fantastic job in her Lilith's Brood series. The oankali, a sentient and highly-advanced alien race, only resemble humans because they take on the traits of the species they are preparing to make first contact with it. In truth, they're covered in sensory tentacles, have three reproductive sexes, and have a greater range of perception than humans. While similar to humans, they are also highly different and incredibly unique, which makes them much more interesting to read about than most other aliens I've seen.
Creating an unparalleled alien race is not easy, and it's hard to expect a writer to make each species he or she creates entirely unique. Nevertheless, there is still a want for more diversity and otherness to our aliens. It shows a real effort has been put in to the world building, rather than the writer slapping on some black eyes and hooves in an attempt to make them different.
Bottom Line: Not every sentient alien has to be incredibly unique, but put a concerted effort into your world building to avoid making boring human clones.