I think mustard from a jar is ok, as long as it's a GOOD quality Dijon mustard. Otherwise, dry mustard.
Curry is a good variation, but it has to be a very light touch.
I prefer a touch of sweetness in my deviled eggs, so sweet pickle relish is good. Bacon is a nice touch. The sweet relish variation is considered to be a Southern thing.
One of the best deviled eggs (well, at this point they're no longer deviled, they're something else), was buffalo style, with blue cheese, finely chopped celery, and bacon. I ate a LOT of those that night.
Another very good variation I've had in the past is deviled red beet eggs. You pickle the eggs, then use whatever yolk recipe you want.