The red dress isn’t tacky because it’s “un-bride like” or “non-traditional,” it’s tacky because I think it’s a fashion fail.
There is an elegant way of showing skin. If her red dress was in another type of fabric (softer, sheer, or flowy), with proper draping, she could have pulled off the high slit. For example, the black gown Angelina wore, considering we ignore her awkward pose. The white dress next to her is a good example too.
I love the lehnga with the slit! I've seen outfits done like that where its like a long jacket type shirt with a slit and straight pants underneath. It looks really good!