The mark of a good bowler is to have a variety of deliveries so that they can vary it according to the weakness of the batsman. Combining this with a consistent line and length will make sure that the runs are kept in check and it will force the batsman to take risks which will ultimately result in wickets.
Another important attribute of a good bowler is to limit the number of no balls and wides bowled and keeping the extras given away to a minimum. Another useful talent is to disguise the delivery, where you have the batsman expecting one type of ball when in reality another has been delivered.
It is also important to vary the flight, pace and spin on each ball so the batsman does not get a feel for the bowling and settle into a rhythm. A bowler also has to be able to react quickly should there be a chance of a return catch or if the shot by the batsman can be stopped by the bowler. A balanced follow through ensures that the bowler is in position to do so.