All of us, as students, have come away from the first day of a new class with the impression that it could be a really interesting class. From a purely practical point of view, the first day of class usually falls within the University drop and add period. If the teacher fails to give students a good feel for what the class will be, students are likely to exercise their privledge to change sections. This may not be a problem if you are teaching a huge lecture class, but it could mean your class will not make if you are teaching a small seminar. Beyond the practical consideration of keeping the students in your class, is the aspect of keeping the students in class enthusiastic and curious about how your course will develop.
Successful teachers offer a number of suggestions for setting the stage that first day.
Of primary importance is the need to be organized. There is nothing more discouraging for students than to feel the first day of class was a waste of time because the teacher did not prepare anything, or because the teacher forgot something he or she had intended to do. Some teachers assume that because drop and add falls within those first few days, they don't want to really start the class to avoid having to repeat important information. This practice in fact penalizes the students who have made the effort to be there, and tends to dampen their enthusiasm for the class. Although you may want to wait a day or two to go over a detailed explanation of the syllabus, do pass it out the first day so students know exactly what the course will include.