I always make my own salad dressing. Nine times out of ten, it's a vinaigrette spiked with mustard. Sometimes, I'll use just a tiny bit of mustard for a background flavor. Other times, I'll use quite a bit so it dominates.
With different types of mustard you can vary it quite a bit. Grainy mustards add nice flavor and texture. Sweeter mustards are nice to use in small quantities. Sometimes I like a horseradish-spiked dijon. Most of the time it's just straight, high-quality dijon.
I have no less than 10 different mustards on hand at any time. Combined with various oils and vinegars, the dressing combinations are vast.
Two other things that often find themselves in my dressings are fresh thyme leaves and finely crumbled queso fresco. The thyme, if used judiciously, nicely highlights a green salad. The cheese has quite a neutral flavor, and adds texture and a bit of creaminess to a dressing. I'll often do this when my salad mainly consists of lettuce and I'm trying to make it a bit more interesting.
Best,
Michael