One thing that never ceases to annoy the hell out of me? Reading a story in which one character feels the need to change who he or she is for someone else, and it's treated like the most romantic thing in the world.
Interpersonal relationships are a part of many - if not really almost all - stories. Books, movies, TV shows, fanfiction... whatever. Not always at the center, not always of the romantic kind, but always there. As human beings, we simply need them in our lives, because we need to interact with others. What always makes me turn away from a story, as good as it may otherwise be, though, is when those relationships - or, to be more specific, one particular relationship - is being singled out as one character's sole reason for living. That is just a very unhealthy way of looking at a relationship, and to me it always feels like it really diminishes the beauty of it all.
I simply cannot read a story in which one charatcer feels like he or she has to sacrifice part of his or her personality in order to keep another person around. If you ever feel like you have to change who you are in order to be with another person - well, there may be something wrong with your relationship.
Of course, certain adjustments are fine. Like, make them learn to chew with their mouth closed, or teach them to not leave dirty underwear lying around on the bedroom floor, or, if it really makes sense in the story, even let them give up their job and move to another town to be closer to someone, but that is already something that has to be handled very carefully.
But the very idea of 'I Live Only For You' is enough for me to close a book, turn off the TV or stop reading fanfiction, because... no. Just. No. Don't ever make one character live for another one. Don't have them give up their own hopes and dreams so that someone else can be happy. They will end up resenting each other for it, even if the part of the story that we get to see ends before that. A relationship is hard work, and even fictional characters deserve to go through that. It is much more romantic to have them explore how they can fit together as two separate people, and to make them reach their own compromises, and have them Stay Together Even Though They Might Want To Kill Each Other Sometimes.
Never define a character as one half of a couple. There's more to a human being than that.
Yes, it's great to read about how perfect two people are for each other. But it's even better to read about all their faults and the things they do that drive the other person up the wall, and how they still stay together.
Perfect people and perfect relationships are boring and unrealistic. It's their flaws and their imperfections that make fictional characters interesting. AND it's the potential for conflict that makes a fictional relationship interesting.
I have no problem with fluff. I love it. But even fluff should never be randomly exchanged romantic nonsense between two empty shells, or, even worse, one great character and one empty character shell. If it's just a small scene, fine, you can probably get away with that. But don't make it the point of a story. Just No.