But, why WOULDN'T aliens be humanoid? There is nothing odd about having sense organs (eyes, ears etc), nothing odd about having an organ to integrate and use the information from the senses (brain), and nothing odd about having most of your sense organs clustered near the integration organ (a head), nor about having that cluster positioned to best utilize the sense organs (a head that is high above the ground rather than below your butt or something if you have vision). There is nothing odd about having manipulatory appendages that are closer to the sense cluster than your locomotory appendages (walking with your feet rather than your hands), and that leaves you basically with humanoids or centauroids... since symmetry is a lot easier to deal with evolutionarily (do this here, and do the same thing over there, rather than do this here, and do that there). I'm surprised there aren't more centauroid aliens, but humanoids require fewer special effects. The only thing left to play with is what the senses are (why don't more aliens have senses we don't, like sharks and electricity, or even something we'd have to evolve a whole new vocabulary for?) and how those senses constrain cognition...