I don't think this is a legitimate government role. Private enterprise does this well enough currently through advertising revenue. The Zwackus model would get us roughly the same content, minus the advertising. This seems to me to miss the opportunity represented by taking out the ads : you can improve the quality of the content as you are no longer subject to the tyranny of ratings.

This relies, of course, on a preachy moralistic world view which Zwackus will undoubtedly jump on heavily...

Since you've already anticipated one of my objects, I need not make it.

However, I do think that "same content, minus the ads" is a much bigger accomplishment than you grant.  The ads are a pernicious evil in and of themselves.  They are actively and positively manipulative and destructive.  

Furthermore, the ad-driven system means that content is consistently targeted to the social and demographic groups that the advertisers find valuable.  So, we have lots and lots of stuff about young people in an increasingly aged society.  Poor people are poor, so it doesn't really matter what they watch.  Etc.

The centralized, tax-supported system would make each viewer equal, in a positive way.  New entertainment niches would open up, and new people and their experiences would be reflected in art and in culture as a whole.  

Much of the material may be crap, but 90% of everything is crap.  This is as much due to the dearth of creativity and the difficulty of the creative endeavor as it is to anything else.