Vicarious ExperienceA good read will let me 'see' or 'feel' what it's like to do things I can never do, go to places I never can visit and - most importantly - live through things that I never can, possibly because they're unreal, in the past or otherwise just not available to me, or simply things that I would never do/see/experience in reality. This, for me, is a key reason everyone should read - to get out of our own experience and to safely taste other possibilities. It's also a key reason that teens (for example) should be allowed their 'edgy' reads - to explore and think about possibilities that actually, in real life, they should avoid.
Emotional and Psychological RealismFor me, a book fails if it doesn't 'ring true' emotionally and psychologically. To sympathise with a character (or to feel ambivalent about those love-to-hate-them characters), they have to have psychological truth. Equally, a situation can be outright ludicrous, but if the characters react to it in a way that feels real, an author can get away with anything.
The Thrill of the Well-Chosen WordYeah, that's right - here's the English teacher bit. You may be assuming that these are the complex and delicious words, the ones we rarely hear, but that's not necessarily the case. In my current read, Pigeon English, that well-chosen word that makes me smile might just as well be a non-standard word like 'hutious' or, quite often, a well-placed 'even' like "It's even too easy." :)
Escape from Mundane RealityWho doesn't read for escapism? Really? To get lost and be somewhere else, in some other world or other time? And yet, at the same time, we learn more about ourselves doing it. Amazing.
The Joy of New KnowledgeAs a kid, I learned so much about the countryside from books like Enid Blyton's, about the world and its animals from Willard Price, and I still love to pick up the odd nugget from books. I tend to get this particularly from historical reads at the moment, and I always have to look up the period when I finish a historical to see where the edges are.
So, those are my top - but of course, not my only - reasons for reading fiction, or things that make a good book good. Would yours be different?