One thing I didn't learn until later in my DMing career was the subject of player expectations. I managed to avoid almost all the speed bumps because my starting group played the way I did. Thankfully they didn't try to walk off with the aliens that were trying to abduct them. But talking about expectations is important. Extra so when you're stepping away from the paradigm of D&D and dungeon crawling. After all if the players don't know they're supposed to be scared of the aliens, they may just take things into their own hands. With amusing or disastrous consequences. Turning your possible Independence Day inspired epic game into a tea and (flying) saucers party like something out of a Daniel Pinkwater book. Now go read Tooth-Gnasher Superflash and be inspired.