If your leggings feel like tights or a piece of hosiery, do not simply wear a shirt and call it a day. These form of leggings are for the lazy dresser or for someone who wants to dress down a dress or skirt. These leggings replace tights, knee-highs, thigh-highs and stockings when wearing a dress, skirt or even shorts. They are sometimes translucent, meaning people can probably see your underwear if you don't cover that area.
If your leggings don't look like pants but are opaque and don't slip as you walk, then wear these with a high-low top or tunic. This is where a lot of the confusion arises. Many women wear these type of knit leggings as pants because they're opaque and fit good. The reason this is inappropriate is because the material fits just as tight so women look like they have camel toe and for a more curvy woman, her bottom is accentuated in a bad way.
The leggings that meet the requirement for pants are the ones that look like form-fitting pants. They are usually made of real or faux leather and rayon. They still fit tight but don't hug your curves so tightly. You may wear these in replacement of a pair of pants although I like to at least cover my bottom half way to avoid the camel toe and emphasized bottom.
FASHION TIP: If you feel uncomfortable wearing your leggings with a shirt then you're wearing them inappropriately.