I've been on only one previous cruise and am going again over New Year's.
If I had to suggest one item to bring it would be a small power strip. There aren't very many outlets in cabins and it's useful if you have electrical items that need to be charged.
Post-it notes are handy - I had a small pack and they were nice to leave notes on for the cabin steward as well as the cabinmates!
I think I'm going to include one of the plug-in nightlights that also freshen the air and have an extra outlet. It will create a nice light for a dark cabin (I'm interior) and have a nice smell at the same time.
I do agree with the duct tape. You can buy a small "folded" pack for about $1.00. The uses are endless.
I agree with the previous person's post that you don't "need" extra items. However, it's "fun" (and somewhat relaxing) to have door and/or cabin decorations. If it doesn't take up much room (and it's legal - i.e. iron vs. wrinkle spray) I say bring it if it makes you happy.
Cruises are a special time to enjoy life - enjoy yours!