The lack of paragraphs definitely plays a part, good writing is very important. Doesn’t have to be crazy flowery prose with no mistakes but just generally good writing and flow is important. Also, if it’s an oc fic, the oc should be kind of developed!
Obviously anything ai is a turn off, thankfully i haven’t come across that yet!
And personally, i like a slowburn for romances! So anything that’s too quick and not developed enough sets me off.