You can read all that here: http://www.riotgames.com/careers/game-designer
But I still quote it.
You Are:
Mission critical: you probably have experience in game development, but even if your previous trajectory was in law, analytic philosophy or strategy consulting, you’re propelled by an engine of critical thought with unlimited theoretical horsepower; you articulate game concepts with balanced persistence, analyze and ideate mechanics with equal and opposite force, and dismantle and assemble systems with both skill and aplomb
A design maverick: you’re fluent in many prevalent game systems and mechanics but have the critical thinking skills to know when it’s time to defenestrate them in favor of something more radical; in other words, you know the rules and know when to break them
Intellectually curious: you’ve most likely earned a degree or two, but know education doesn’t have an end date—you think on your feet and absorb new knowledge every day; you relish challenges for their learning opportunities, and know when to pummel a problem solo and when to recruit expert backup
A seasoned summoner: your match history in League of Legends would fill several large scrolls, and you’re a wizardly scholar of gameplay elements and how they fit together; you’ve played titles from every genre and your typical post-game ritual, win or lose, has more to do with dissecting design decisions than sizing up your final score
To the point: your verbal and written communications are crisp and clear, distilled from complex ingredients into smooth, yet potent, shots of insight; your best work comes from a place of collaboration and teamwork and you know that quality results come from dynamic communication throughout the design process
Tech-friendly: you either have a technical or programming background, or you think like an engineer and attack problems with the same systematic precision; experience with scripting languages and designer tools is a definite plus