Nick's open-minded to the extent that he takes people pretty much as he finds them. He doesn't seem to be infected with the kind of racial prejudice displayed by Tom Buchanan, nor the rampant social snobbery that ultimately prevents Daisy from walking out on Tom for Gatsby.

That's not to say he's not judgmental; far from it. In fact, if he weren't able to pass judgment on all that goes on around him, Fitzgerald's undisputed masterpiece would be much the poorer. The simple fact is that we, as readers, need Nick as narrator to be judgmental. He, after all, is our way into the story, our guide to a world of opulence and glamor that most of us have never experienced or will ever get to experience. If Nick just blindly went along with what everyone else was doing without passing comment, it would be much more difficult to make sense of everything that happens.

So although Nick is indeed open-minded, both in relation to other people and to new experiences, that doesn't mean he won't evaluate them according to his own moral values.