The barrier can be of their own making as with the famous Pride and Prejudice or something real like Jane Eyre. One way or the other, the lovers won't find it easy and sometimes, rarely as with Gone with the Wind, it does end without the happily ever after, but that's a rarity and frankly-- was it really over between Rhett and Scarlett?
For my own favorite romances, there is one character who is mythic. That person is bigger than life while the other plays the part of the rest of us. The real person is sucked into the drama through a gatekeeper experience. Something comes along to turn them from their average path-- and that something might be meeting that mythic person.