Call them motorcycle boots, engineer boots or work boots, they're all a little clunky, casual, and can completely alter your look. A mini dress that says "I'm hot" suddenly seems irreverent and maybe even a little rebellious when paired with gray-leather, slightly slouchy, mid-calf biker boots.
Use a pair of tough-girl boots to dress down a skirt or edge-up a pair of jeggings. (You may find they're easier to walk in then 4-inch heels, too, which is nice.)
And should those tough-girl boots have studs, laces, or buckles, all the better.