Did something pop up to you either in game through trial and error or through a post you read here on Zam that you didn't know about when you first started playing? Share the wealth my friend and post it here!
I'll kick it off. I didn't know until almost a WEEK AGO!!! (in caps to indicate my ignorance :D ) that when you summoned and dismissed your retainer from the bell in the adventurers guild that it never went to an area in which it sold stuff for you. It just went... away. Once I learned this and physically placed the retainer in a ward (this was right before the categories came into play) imagine my surprise that I couldn't get to her anymore without going to the particular ward to see her again.