Another constant is conflict. It's what exposes every characteristic and drives the story forward. Conflict doesn't mean war, battles, or anything of the sort. It could be an internal conflict, a problem in their past resurfacing to challenge them now, a long time dispute, and lots of other things, but the main characters need conflict and eventually resolution. It helps readers sympathize and gives them warm fuzzy feeling at the end. Also conflict between the two characters falling in love...which they of course will eventaully get over...but it's a lot more interesting to make the reader wonder.
Another, of course, is strong, believable, distinct characters. Characters who have their own manner of speech, body langauage, habbits, and such. They must be real to the reader (and the writer for that matter). I'm sure there's more, but it's 6 a.m. and time for my second cup of coffee, so that's all I can think of right now.