Why is Quinn good top?
What sets her apart from other ADCs and let her be viable top?
What sets her apart from other ADCs and let her be viable top?
as a bonus to what other people said, she's pretty good at split-pushing
Q- Cz who doesn't love blinds? 
W- A pretty good scouting tool for incoming ganks, and a good attack speed/movement speed steroid for your Harrier.
E- A good disengaging tool if get dived into, for example, a Jax wanting to commit by going EWQ and then you E to get off from him. Also a pretty annoying sticking tool when your enemies wanted to escape.
Quinn have her e She can stop vi'q velkoz's r and many other skills shes realy fun champ 😍
because that's the only role she isn't complete garbage
This is my general Quinn build when I play her toplane.
or
(Depends on enemy team comp)

You still deal a lot of damage (especially with Harrier mark + Infinity Edge) while having some tankiness on you. You will do very well against most of the melees in the top lane. Your Vault also lows you to back off when they come in close. She does a lot better in a solo lane than as an ADC imo.
shes good top because she has very good single target burst/ focus (also why shes not a very good bot lane adc pick) her blind and vault aslso make her very safe laning. And her free vision also allows her to not rely on support warding as heavily. (but she still needs to ward to not get shrekt by jungler)
It's mostly her vault. She can reliably get away from the bruisers that tend to thrive top lane. Unless their jungler is a Vi or Sejuani with ult, ganks are going to fail because of your vault+valor. Your blind is really icing on top of the cake in terms of safety, and it's longer range harass if you're not doing as well in lane.
She also has much higher damage earlier in the game than other ADCs. Plus her ult is fantastic for having great map presence. You can use it for a quick gank to mid and then TP back to lane.
It's also good that she synergizes really well with Triforce, because you really don't want to be building exactly the same thing as your bot lane marksman.
In my opinion Quinn is good top lane because...well her escapes and her vault. I like to play her top lane because a lot of the top laners are bruisers and Quinn gets away easier if nearly dead.
Quinn top is like Teemo top. Heavy zoning early game but late game she does nothing because she isn't a tank/offtank and she's just another ADC that the team needs to protect, more of a liability than anything else. Maybe it works now because you are playing against idiots, but any half-brained top lane player will let you have the lane until they get
or
and once they have that they will sh*t on you in all trades and there isn't anything you can do about it because you have no tankiness and can only use basic attacks.
And forget about it if you are playing against an assassin top like
, if they know what they are doing they will get first blood on you and kill you over, and over, and over, and over again.
At least Teemo can give his team free vision with the mushrooms.