The lack of build up is what really sinks the pairing for me. Couples like Hermione and Ron worked because they built up tremendous platonic chemistry before they got together. So Rowling didn't feel the need to remind us that Ron and Hermione were "meant to be" by having them declare their eternal love on every other page.

Harry and Ginny, on the other hand, didn't have many buddy moments before Harry jumped her in the common room. So their romance felt really forced and unnatural.