Asking people to judge the length of lines is an insignificant task so pp's were probably willing to conform. On a more important task we would expect conformity levels to drop. However pp's had to answer out loud in front of strangers and so they would have wanted to be accepted by the group--- The findings may, therefore, only tell us about conformity in certain situations.
DECEPTION AND LACK OF INFORMED CONSENT: pp's didn't know the real purpose of the experiment or that other pp's were confederates of the experimenter. However the true purpose of the experiment could not be told otherwise the experiment would have been pointless. These ethical issues can be overcome because researchers did properly inform the pp's after the experiment and offer them the right to withhold therir data.