Different people like different styles of writing. For example I love to read, but I thought Lord of the Rings was boring and badly written. I am sure someone who likes that style however could tell me in detail why it's genius.

If mass appeal doesn't count as a method of measure, why does quantity of books count? I have read hundreds but still don't like some of the greats.

Reading a lot of books you already consider to be great is rather a self fulfilling cycle. It it's not in that style it cannot be considered great.

A lot of people like to say Rowling is crude (after all it is a childs book) however great complexity of writing can easily take away from the quality as it makes reading a chore not a pleasure. Not all characters have to have immense depth (and those who have depth still get criticised for being predictable or not deep enough).

For me the originality of the story, the interwoven characters (though I wish books 1-3 would be re-written with all the original content), the sheer grip the book holds on the reader and the amounts of research that went into the story are what make it a great book. For example the different terms for spells are based on different languages depending on their use.

Household spells tend to be based off of Old English words, because English household words are usually rooted in Old English. More business like and grand spells are based on Latin, as our business words are often rooted in Latin. The same applies to Greek and medical spells.

At the end of the day everyone has their own opinions and no matter how much you read not everyone will agree what makes a good book.