Check out this letter from the publisher of a magazine you've never heard of to the founder of a little magazine called Readers Digest:
But, personally, I don't see how you will be able to get enough subscribers to support it. It is expensive for its size. It isn't illustrated... I have my doubts about the undertaking as a publishing venture.
Of course, he was right--given his assumptions. And that's the except part.
Criticism of your idea is usually based on assumptions about the world as it is. Jackson Pollock could never have made it as an painter in the world as it was. And Harry Potter was rejected by just about everyone because for it to succeed the way kids read would have to change.
The useful element of this sort of criticism isn't that the fact that people embracing the status quo don't like your idea. Of course they don't. The interesting question is: what about the world as it is would have to change for your idea to be important?
In the case of Readers Digest, the key thing that changed was the makeup of who was reading magazines. Most of the people (and it was a lot of people) who subscribed to the Digest didn't read other magazines. And so comparing to other magazines made no sense, except to say, "this is so different from other magazines, the only way you're going to succeed is by selling it to millions of people who don't read those magazines." And Starbucks had no chance if they were going to focus on the sort of person who bought coffee at Dunkin Donuts or a diner, and the iPad couldn't possibly succeed if people were content to use computers the way they were already using them.
Keep that in mind the next time a gatekeeper or successful tastemaker explains why you're going to fail.