According to the CIA, Guatemala suffers from two major issues: human trafficking, for forced labor and commercial sexual exploitation; and transiting illicit drugs, mainly cocaine and heroin. It's unlikely that travelers will come across either of these, but petty crime is surely an issue, especially in the major cities.
Watch the news, ask the locals and listen to other travelers while in Guatemala. Your safety is in your hands, and many problems can be avoided by remaining cautious at all times. Generally, it's recommended that people don't walk in unknown neighborhoods at nights. And hiking alone or in small groups can be dangerous as well, with the often-present threat of robbery.
Read the forums and blogs, but don't believe everything you read. The trick is to find trustworthy commentators who post often and have many good reviews. They will often be experienced travelers who are still on the road.
