You never have a second chance to make a first impression," so goes the saying. With the exception of excessively oversized classes, few teachers will be pleased with a sharp decline in enrolment after their first meeting with students. That's why we sometimes go out of our way to plan on an impressive first class: we want to appear kind, easygoing, humorous, understanding, inspiring, well-organised… in a word, everything a "good" teacher should be like.
But mind you, a perfect first class may get the course off to a good start, but it may also create false expectations and throw a shadow on the days to come. If a quick sense of humour is not your strength, it is perhaps unwise to crack a few jokes on Day One just for the sake of a good first impression. If we don't plan to sacrifice Sundays answering phone calls and emails, we'd better tell students at the outset that we only work on work days, so please send in any inquiries early. And in a similar vein, if we're committed to active learning and expect considerable class participation, we should also start from Day One! In a word, "what the class is like later", rather than "what the students might like", should be the golden rule of an efficient and helpful first class.
Of course, there is something we should all try to achieve in this important first contact: we want to show our passion for the subject, getting the students interested in and prepared to work hard on it. But in terms of how to achieve that end, teachers' approaches may vary. After all, in teaching as in everything else, there is no single best way but only the way that suits you best.