Every deli has coleslaw. Not every deli has good coleslaw. The difference comes down to one thing: restraint.
Too much mayo and it's soup. Too much sugar and it's dessert. You need just enough dressing to coat the cabbage, and then you let the vegetables do the talking. Fresh carrots, a little red cabbage for color, and a splash of vinegar to keep things honest.
My mother's recipe had celery seed in it, which I thought was weird as a kid but now I wouldn't make it any other way.