Re: New instructor on the block
In all cases (e.g. 99.9% of the time), don't EVER give voice to value judgements about someone else's actions (like "your rolls are good"); instead, point out the specific details you observe: "I see that your backfalls are smooth and you look comfortable with them. Many people have some difficulty there." (Or skip the second sentence, depending on the circumstance.) This practice trains your own powers of observation, and that of the students as well. It also generally amounts to more meaningful and helpful feedback. And people can provide their own value judgements, but they will always appreciate the attention you give them.
On joking - be yourself, but elevate the level of decorum and compassion, in keeping with the fact that you are now the face of a tradition or organization that you value. Be genuine, aim to express the best of yourself and of Aikido, be aware that you WILL be misinterpreted, probably quite often; but if you always act for a place of genuine good will, that will come through in the end.