I imagine most items will be logical, but here's what I found I either overpacked/missed/or found to be a must on my 4 cruises:
Overpacked:
Way too many pieces of clothing. On the ship you are usually in a bathing suit & coverup during the day & dress nicely @ night. Too many pairs of shoes. 1 ea. of beach/pool, dress up, casual more than enuf. Jewelry (only nd 1 pr earrings & 1 nklc - no one notices, anyway). I even packed way too many clothes for the kids, too. Mix & match - that's the key!
Absolute necessities (besides the usual personal items):
Sweater for cool dining/enter. areas
Cameras (incl. underwater) Strong suntan lotion
Sound conditioner (white noise machine) to drown out people in the halls, next door, ship noises (esp. great for the young kids)
Fanny pack to carry your itineraries, key card, etc. in
Missed:
Extra room left in the suitcase for purchases.
That's all that comes to mind right now. I'm sure you'll get a lot of great responses to this.