Most of the items below have been the cause of disagreements on trips I've been part of, and it wasn't always pleasant. Other items were sent to me by people who read this document on an email list. I've been on both PCS and SPS trips where a driver decided to go home early, stranding a passenger. I've been on trips where people agreed to share gear, but refused to EVER carry it. Once a bear peeled a car door back a bit just to verify that we had removed all the food.
People show up with inadequate food because they thought the leader provided it.
If you go with the same people all the time, you may not need to do this. It will also make people uncomfortable to talk about it but better uncomfortable than angry and upset when a misunderstanding of expectations occurs. I will very rarely share any gear with anyone, except perhaps a rope. That way I don't get forced into abandoning a trip when the person I share with wants to leave. I recently climbed Mt. Shasta and one of the climbers was in a hurry to get back causing me some extreme fatigue and some real anger and providing him with only a minor convenience.