Simply because the design is different from the standard doesn't mean it is bad. It is an attempt at changing the meta, just as Xypherous had done with Riven's design.
Quinn's design is that of a dueling, assassin AD carry. Nothing about her says, "This is a champion you want to pick for teamfights."
The AOE on her Q is minimal. Her passive only works on one enemy. Her E chases down a single target. Her R gives her increased fighting capabilities on one target, although the second cast makes it a nice teamfight finisher.
From what I understand of her design, Quinn is a mid-game champion that excels at pushing her advantage when you have it. She has great laning potential and is better off as a kill lane than a simple "farm to power" AD carry. Her design appears contradictory only because of the static and closed definition of AD carries. That is why she is designed like a bruiser, because the mindset is meant to appeal to people that play bruisers now playing AD carries. Like bruisers, both have mid-game power and strong laning phases and more focused on beating down the enemy more than simply farming.