As a St. Louis resident, I know that St. Louis is just like any city: you need to know where to go. As long as you don't go onto the Eastern side of the river, you're generally good. That said, it is a city, so you always need to be careful.
Here's an article my school's newsmagazine did this year on crime in St. Louis, if you're interested:
http://www.chsglobe.com/cover-story/...angerous-city/