an open network allows people to access your computer just as if you had them hooked up to a LAN. If your computer itself is secure, you should be safe.
If its a home network you didn't bother encrypting freeloaders could slow you down and potentialy get you blamed for illegal activity. the brat next door could crank the P2P, then when the fuzz shows up with a warrent, better hope your home computer is all legit.