A queen's husband is the Prince Consort, but a king's wife just takes on the feminine version of her husband's title despite being his consort. Kate is generally considered the future Queen because most people think of William as the next king rather than his father (really depends on Charles' decision to take the throne or not since he's first in line). In Diana's time, the Queen wasn't so old, so she wasn't called the future Queen. Now, Elizabeth is quite elderly so people are more actively considering who the next monarch will be. Charles' current wife hasn't taken the title of "Princess of Wales" because it was last held by the late Diana. After the divorce, Diana was still called the "Princess of Wales" because she was Charles' first wife and the mother of two princes of Wales. If Charles ascends to the throne Diana will still be Princess of Wales and the same goes for if William or Harry ascend to the throne, however, if William or Harry ascend to the throne she will also be the late Queen Mother. "Future Queen" is just the media.
I don't really pay all that much to the royal wedding, but I make a point of researching monarchies since I've turned my love of history and English into careers (teaching history + writing fantasy novels).