I think maybe there should be some generality to the aliens so as to make them necessary. If say a character is acting in a way we all could, then why create that species in the first place? Why not use a human or any number of other aliens?
At the same time, we should observe that many of the humanoid aliens we've seen should realistically have a wide breadth of characteristics, some of which will overlap with other humanoid aliens and humans themselves. This makes the job of the writer more difficult, but 1) done well, they would get that much more glory for it, and 2) it just makes sense given the similarities between aliens we've seen.
I'm usually most impressed by writers who create new ways of being and new forms of being (non-humanoids) well. If we ever find real aliens, I hope they're very different from us and in studying them, we learn not only more about what it is to be "human" beings, but beings of the universe, period.