I don't fully agree the other definitions, so let me throw in mine.

A heartbreaker is a system deriving heavy inspiration from another system. It's often characterized by having a few really cool ideas buried under a pile of bad or poorly thought out mechanics.

The "heartbreak" is that these creative nuggets are forever lost under the bad mechanics resulting from the authors inexperience or inability to grasp why a mechanic was done that way in the original.

A "Fantasy Heartbreaker" almost always refers to a heartbreaker derived from D&D.

In the last two years I've seen the term used without the negative connotation. This is baffling to me - having your RPG called a "Heartbreaker" is not a compliment. In what world would you feel happy being told "Your RPG breaks my heart?"