What things can a doctor tell about your heart by just simply listening to it with a stethoscope?

They are looking for irregular beats, skipped beats, extra beats, and other anomalies that can or will cause serious problems.

When you listen to a persons heart, you are listening for two things. 1. The closing of the valves and 2. turbulent blood flow. The "Lub Dub" sound the heart makes is actually the sound of the valves closing against the backflow of blood. If the heart's valves don't close correctly you will hear turbulent blood flow, this is what is known as a murmur. There are also some things about the rate of the heart and the closure of the valves that the doctor can tell simply by listening. The major purpose of this is to see if anything is out of the ordinary. If it is then he moves on to diagnostic testing such as the EKG and blood work.

I guess the best way to explain it is that He cant tell a whole lot at all from listening to it except that theres something else going on besides normal operation and he should do some further testing.