Style is always subjective and comes down to personal choice. Some people may choose something very traditional that blends in perfectly with their home's style and neighborhood's feel, while others will want to make a bold statement with a fixture that stands out dramatically.
Narrow down your style
Choosing a style that matches or blends well with your home style will give a more cohesive look to your home. A more modern home might look odd with an old-fashioned stained-glass style fixture and a nautical themed light would seem out of place on a southwestern styled stucco house. If you are not sure what style your home is, take a walk around your neighborhood to see what you like (or don't) about similar homes.
Add your personal style
If you are the homeowner, you will be seeing and using this light fixture more than anyone else, so the style must be something you are at least comfortable with, and hopefully love. As you browse options, imagine how each of these fixtures would feel greeting you each day as you return home. Pick something that will give you a feeling you enjoy!
If resale value is a consideration for you, selecting a more timeless design is a safer choice. Look for an outdoor flush mounted fixture that would look appropriate on almost any house on your street.
Selecting a finish
Although they can come in any color, most outdoor fixtures have a metallic finish:
Silver colors, like brushed steel, platinum, or nickel, tend to look more modern.
Gold and brass colors are more traditional, while plain white or black are usually the most unnoticeable, for when you want something to just blend in and not stand out.
Dark finishes, like rubbed bronze, are often the most versatile, blending well with many styles and colors.
Consider the color scheme of your home as well. You may want to choose a cooler color like silver or white if your home exterior is in the blue/green family, or a warmer color like brass or oil rubbed bronze if the exterior is in the orange/red/brown family. Of course, contrasting colors can make the fixture stand out and be more visually interesting.