I will continue to re-write this until I am better able to articulate what I think.
A good book is a book that is difficult to read sometimes (& not because of vocabulary or grammar) that then rewards you at the end.
A great book makes you want to read it again, no matter how difficult it was. And a fantastic book actually gets BETTER as you read it again.
To me, the worst type of book or story is one that makes me struggle through it, only to find disappointment and despair at the end of it.
Replace "book" with "story" or even "TV show" or "movie"...
And this is not necessary, but my own personal criteria is that I PREFER something that makes me feel GOOD and not BAD afterward.
Credit to Matt K's friend Matt H for saying something similar to this that I have tweaked for my own purpose:
If something makes me feel sad or something afterward, then I still consider it to be a "good" book (as in, well-constructed, definitely has merits, and perhaps meets its goal of affecting the reader, has made me feel something) but I would vastly prefer to feel good; and to me, a GREAT book makes me feel a positive emotion--especially one such as HOPEFUL. Triumphant is good, too. :)
Whether or not art should/has to/doesn't have to/etc. "Send messages" or not is another subject to be discussed...though I'm afraid to tackle it.