There are 2 problems with on board entertainment as I see it.
1. You have no idea ahead of time what you will get.
The lines should provide information on the scheduled entertainment and use it as a marketing tool. I wonder how far in advance they know about featured artists being booked. I would rather book a trip with John Pinet (saw on last cruise - never laughed so hard) than with Marty Allen (A man who should have retired a while ago). Its a total crap shoot.
2. Diverse crowd limits what line can do.
The shows are doomed to failure due to the cruise line's attempts to please everyone with 1 production. With such a diverse crowd, straight down the bland, uninteresting middle is the only place they can go to some extent. They would be better served to divide the huge theater spaces into several smaller venues. A comedy night club, a performing art space, and a traditional play space. The Voyager ships pretty much have the space and are heading in the right direction. There is an ice show, several music venues, and traditional production numbers. They should however offer more alternative productions in studio B. By offering multiple choices, people can select more tailored entertainment options and they could do away with trying to please everyone with a single show.