Well, the other obviously-a-bridesmaid-dress dress was deep red/maroon. So I don't think it's the pastel. It's the solid color.And the "I'm part of a color palette" color--the unusualness of it. And the fabric--few dresses in the "evening wear" section of department stores, etc., are that safe satin/taffeta material.
I like the jeweled belt idea, but you have a separate bodice, actually, right? Or a bodice that's made to look separate? And a tie in the back? You could snip the tie and add a belt, even over a bodice,
Basically find some way to add pattern, color, embellishments that will look more like a true evening dress. Maybe look at some department-store websites, or similar, at their eveningware, and see what makes those dress not look "bridesmaid-y," and then see if you can figure out how to add that to this dress.