There are a few areas considered by locals to be dangerous: Liberty City, in northwest Miami, Little Haiti and stretches of the Miami riverfront and newly-gentrified Biscayne Blvd (after dark). Obviously, in these and other reputedly 'bad' areas, you should avoid walking around alone late at night, use common sense and travel in groups. Deserted areas below 5th St in South Beach are more dangerous at night. In downtown Miami, use particular caution near the Greyhound station and around causeways, bridges and overpasses where homeless people and some refugees have set up shantytowns.
Natural dangers include the strong sun (use a high SPF sunscreen!), mosquitoes (use a spray-on repellent) and hurricanes (between June and November). There's a hurricane hot line (tel: 305 229 4483), which will give you information about approaching storms, storm tracks, warnings, estimated time till touchdown... all the things you need to make a decision about if and when to leave. That said, over the years, many Miami hotels have learned how to lock down, board up and keep their customers safe during Hurricane season.