In 1927, when Clara Bow established her nickname as The "It" Girl, I'm pretty sure "it" meant sex appeal—but you couldn't say "sex" openly in those days, so they called it "it." Sex was something only doctors wrote about, and their books were sold under-the-counter in plain brown wrappers, and even so, the language involved a lot of circumlocution.

Around the same time, Mae West wrote and starred in a play called "Sex" and it was closed down for obscenity.

I don't think there was ever meant to be any mystery about what "it" was.

Today you should be able to say anything you want in plain language, at least if you're trying to explain why you like a certain performer.

Of course you might not know the correct words, but that's one good reason to read criticism.