The reason why the singular might be used is that, in your example,"Its huge structure and complexity" might seem to be a single quality of "it" rather than a list comprising two items. That is more likely to occur if the qualities have some kind of coherence. In this case "huge structure" and "complexity" could well be considered a single feature rather than a list of two, because huge structures are often complex.
If you wanted to say "Its red colour, and the music it played..." it would be very odd indeed to use the singular verb "was".
In such a sentence you would never be wrong to use the plural form, but you might find the singular in the conditions I have described.