Avoid the "driving" melodies associated with other genres. Emo, while occasionally a faster rhythm, is less rhythm driven than a lot of other genres. In emo love songs, the singer takes time to develop emotive qualities.
Add some thin guitar. A trademark of many emo love songs is an organic guitar sound that resonates with listeners as a relaxing sound. You get this by unplugging and playing the strings of the guitar with less force and more nuance.
Find your emo voice. The emo song relies on emotive qualities that listeners should be able to hear in your voice. Not every emo singer has to sound like Bright Eyes, and a melancholy, breaking sound is not a must for emo, but you should be able to sing without glossing over your vocal style so that what goes into the tracks and comes out of the speakers sounds organic, natural and vibrantly present.
Mix inventiveness with attention to useful conventions. You'll want to develop your own style without disregarding things like appropriate instrumentation, track length, selections of verse, chorus and bridge segments and other qualities that help identify a song. A lot of emo and other songwriters disregard these standards at their own peril.
Talk about love with irony. Rather than being glorious or excessively sanguine, as the old-style love songs usually are, emo songs tend to lean more toward understatement and, at times, even sarcasm. Pay attention to how you use your love ballad in writing emo to avoid saccharine, sappy lyrics.