Good old-fashioned regular TV would be great. I don't know if it's just Carnival, but having just 2 channels that showed mostly old sitcom reruns, decorating shows and cooking shows from the major networks' sattelite services was a drag. At one point on the Paradise, I turned on the TV and found both channels showing Judge Judy... Nobody goes on a cruise primarily to watch TV but having a bit of selection for those times you want to watch TV would help. I'm sure it's just a matter of the cruise line not wanting to pay for more, but it would have been appreciated.
CNN would be good too, as I found myself feeling very cut off from the world while on the ship. The ship's news service consisted of text only highlights that you had to actually sit and read, with a latin music soundtrack. I like to listen to CNN while doing other things, and really missed the commentary/discussion aspects. Perhaps most people like to be cut off from the world while cruising, but they wouldn't have to watch that channel.
I also second the suggestions of online account access and internet access. (Though on a separate note - if the cruise line is providing internet access, then internet access would be appreciated. Carnival's service is extremely limited - AOL access for people who use that but otherwise just a web browser and the option of $4/msg email service if you don't use hotmail, etc.) Then again, I'm probably not the target market in this area, since most people probably don't take their laptop on cruises.