Like any large city in the world, Houston has its share of unsavory characters.  If you use common sense, you'll enjoy your trip and have no problems.  A few tips:

 

Don't turn left in front of a Metro train.  The no left turn signs are there for a reason. Silly people don't follow them and end up on the nightly news.  Don't be silly.

Stick to well lit areas at night. Goes without saying, doesn't it?

Walk only where you see other pedestrians.  Don't be the only one on the street.  Houston isn't a walking-around kind of town.

Don't flash your cash.

Don't step out in front of cars in a crosswalk.  Keep in mind that Houstonians drive aggressively.  If you're a pedestrian, don't automatically expect drivers to stop for you.  It may be the law in other states, but it could turn you into a speed bump in Houston.

Don't show your road rage.  The driver next to you could well be armed.  If you're irritated at another driver, don't give them the old "one finger salute".  It could get you in real trouble.