Trope: Amnesiac Lover
Description: In this trope, our love interests have already found each other and have started on the path to "happily ever after". But then...tragedy strikes! One partner suffers some injury or other experience that results in the loss of memory and can't remember his/her true love. (In some cases, both partners have amnesia.) Throughout the story, they set out to rediscover their love. The Amnesiac Lover learns why his/her partner was so awesome to begin with and they resume skipping down that HEA road.
Examples: Sarah from Chuck (yes, again!), Paige from The Vow, Joel and Clementine from Eternal Sunshine of the Spotless Mind
Pros: If you're a fan of the idea that "love conquers all", this is a pretty good trope. Nothing will ever keep our heroes apart, as even amnesia is powerless in the face of true love. This trope also offers up a good change of pace in the romance genre - in a way, we already know the end result for the couple (as they've already wound up together), but we're able to relive the journey with a different sort of tension.
Cons: Most stories that use this trope do not portray amnesia with any sense of medical accuracy. (Most, not all.) If this comes up in a series of books, TV shows, or films, the audience can feel cheated by having the couple they've been rooting for all along be torn apart by yet another dramatic twist. Even if the Amnesiac Lover recovers and the couple falls in love all over again, we're left wondering - are they really going to be the same?
Would/Did I Use It?: While I have included an amnesiac character in a book, she (Liora from EotS) doesn't fall under this category, as she fell in love after the amnesia kicked in. However, I'm not opposed to this trope. Sometimes it's interesting to see a romance from the other end, after the couple has already gotten together. In real-life situations, even without any sort of trauma, I think it's important every now and then to remember the things that attracted you to your partner in the first place. Stories like these can be a needed reminder not to take them for granted.