There is no such thing as "good music" because "good music" is subjective, therefore the personal taste of a DJ is completely irrelevant when it comes to their skills and talent. It's not WHAT they play, it's HOW they play it. A good DJ would never close themselves off into a box and refuse to leave. From hip-hop to rock, pop, r&b, house, lounge whatever it is the larger the range the better. Having the ability to adapt at any given moment requires a great deal of aptitude and it takes a master to pull it off with grace. HOWEVER the single most significant quality of a great DJ does not require any experience. With out this characteristic, no matter who they are, or how long they've been doing it a DJ is worth nothing. It comes down to a very simple but strong desire to connect with the crowd and then followed by a simple desire to want to be great for this crowd.
Whether the crowd is "cool" or not (the subjective interpretation of) "cool", a great DJ will give themselves fully to everybody and anybody regardless of the superficial boundaries they might assume exists. It's building a relationship one song at a time, earning trust, and being completely aware of the energy of the room.
Anybody can play the songs they love, but only a really great DJ can make anybody love the songs they play.