Personal security, including theft, is not a problem in Croatia, but the former confrontation line between Croat and federal forces was heavily mined in the early 1990s. Over a million mines were laid in eastern Slavonia around Osijek, and in the hinterlands north of Zadar. Although the government has invested heavily in de-mining operations, it's a slow job. In general, the mined areas are well signposted with skull-and-crossbones symbols and yellow tape, but don't go wandering off on your own in sensitive regions before checking with a local. Never go poking around an obviously abandoned and ruined house.
Croatia is not unduly burdened with biting or stinging creatures, but mosquitoes do abound in eastern Slavonia, and sea urchins are nearly everywhere along the coast.