Literal translation: "What does it have of incredible?" Translation: "What is incredible about it?" This statement needs the "de" (of/from) because otherwise "incredible" would be a pronoun. Another way of asking that same question is: "Que lo hace increible?" Translation: "What makes it incredible?"

BConon's answer right there is on point, tho I understand how that is something you wouldn't say in english. So adding to her answer, a way to understand it would be... say a friend tells you something, one of them crazy stories and you know he's exaggerating things so you dfon't believe him and then you say "So? What about it?" to him, kinda sarcastically not acknowledging his incredible feat. That would be when in spanish you could use "Y eso qué tiene de increíble?" Or you could say it when he tells you a story like it's epic when it's really not, and then you would actually mean it, there's nothing incredible/amazing/epic about it. The "de" in the phrase is so the world "increíble" affects the word "eso", so "eso" is supposed to have been "increíble", it is the subject. "What's so incredible about that/it?" Hope this helps.

Easy "que tiene de increíble?" = Mmmmm So. y por que aqui hay "de" ? = And that.