Apart from Quintana being a natural talent from day one and having the perfect build and riding style for a good climber he's also just a climber, and his good results come pretty much exclusively because of his unique capabilities in the mountains. He can time trial fairly well, especially on courses with hills, but on the flat he consistently gets dusted by the TT specialists, unlike a couple of other guys.
Not saying I'm 100% sure Quintana is clean, but there's a bunch of riders out there I suspect more than him, despite him being so dominant in the mountains. His story up until this point makes it seem probable that he could be one of those once-in-a-generation climbing talents. I can't say the same about a lot of the other riders who show up at the top of the GCs in the big stage races.