Mostly you should have little issues. The vast majority of open wireless is because the people running them are too lazy to secure them.

Anyone can see the traffic between your machine and the internet which may increase their ability to attack your machine and gain access. Using open wireless just increases the number of people that attempt to hack since once the data gets to the internet it is just as open. It just becomes even more important to keep your firewall rules on and keep your machine patched.

HTTPS is mostly secure but allowing someone who may not be trusted , ie the owner of the open wireless, does allow for attacks. This is the classic Man in the Middle attack and if you are not observant it could work. HTTPS uses certificates to defeat these attacks so you should always think about this if you get any of the invalid certificate warning messages from the browser. You also need to make very sure you are always running https since someone could easily make HTTP pages that look like the HTTPS pages.

The person most at risk are the fools that leave their wireless open since they usually also too lazy to keep their machines patched. I suspect one day more ISP will drive around and find these and send letters with large fines for violating some clause in the fine print related to unauthorized sharing.