I have hosted enough beginner friendly trips or seen my share of oversized backpacks to know that people don't know what to bring into the backcountry. Like I mentioned in my Gear List post you should know after your first few outings what you have not used and can leave behind. But sometimes (nerves involved) you can get carried away and end up bringing more then you need. I have a terrible habit of over packing for cold weather trips. For some reason two buffs, a baseball cap, a skull cap, a baclava, and a heavier merino wool hat always seem to find their way in my pack. I am yet to be on a trip where those items were used altogether. Honestly I can't think of a time I needed all those items and didn't have them (probably because I packed them, like a newb). Below is my list of what I have observed from those trips and some reasons why you may decide you need them.
When in doubt, leave it out. Pack light, hike happy.