What kinds of things happen when I link Steam and Facebook?

This Steam Facebook FAQ seems to answer most of your questions.

When you link your Facebook account to Steam, Steam tries to find if any of your Facebook friends are also on Steam, and suggests that you add them. You also get the option to invite your Facebook friends to link accounts as well, via a wall post, but that can simply be declined.

Your Steam friend limit is also increased by 50.

The support article explicitly states that nothing will be posted to your wall automatically:

Are you going to post status messages to my wall?
Steam will not post anything to your Facebook wall without your permission.

Linking Steam to Facebook will not post anything to your wall. The only thing it enables you to do is find your Facebook friends through Steam's friend system.

Now it is easier than ever to find your friends on Steam. Just link your Facebook profile with Steam and see your friends who have done the same. Don't worry, we won't post anything to your Facebook wall without your approval.

Source: http://store.steampowered.com/news/5644/