Others on the unsecured network could:
Read your incoming and outgoing POP3/SMTP email and it's passwords (email passwords ar not encrypted--passed as plain text). Very few ISP provided email is tunneled through SSL.
Read your FTP passwords.
- Change your router connection settings.
- View your shared folders.
- Generally invade your privacy, from reading your LPSG HTTP headers to reading your blogs.
They aren't very likely to get into your non-shared files, unless they are very good hackers. I personally wouldn't worry much about this. Mostly people just steal bandwidth.