Budget constraints aside, Star Trek aliens were used as a prop to explore extremes of terrestrial human behaviour and culture from an outside perspective - take a human and remove emotions and have them governed purely by logic, you get a Vulcan. Take a human and exaggerate their tendencies towards violence and honour, you have a Klingon, and so on.

Even in the original series there were truly alien aliens - for example the Squire of Gothos was a being of pure energy who just assumed human form, Devil in the Dark had a silicon-based creature, and the Tholians were an intelligent crystalline species who could only exist in high temperatures.