Pros
The people who work here are incredibly passionate about video games. Many employees eat, sleep, and breathe League of Legends or other PC games, and the "player perspective" that Riot has as a result of this truly allows us to connect with our players (customers) on the deepest level. Incredibly open, laid back culture. Things aren't sugar-coated, and you have to have thick skin to work here, but what this means is that if you're doing something that isn't working or is at odds with the culture, you're able to course-correct quickly. "Gamer" humor is injected into most company-wide communications, which helps add to our mantra of "not taking ourselves too seriously." Cross-disciplinary teams are the norm at Riot, and is one of the most remarkable things about working here. As QA, I have direct access to Design, Engineering, Art, and UX, and can talk to any of them with ease on a daily basis. It prevents scenarios where things are worked on in a vacuum and "tossed over" the wall for QA to check. We're redefining what it means to have test-driven development, and the result on the holistic product quality shows. There's no shortage of work to be done, and if you're a go-getter, you can rise quickly at Riot if you're delivering results and solving perceived problems in ways that add player value. I've been here for quite some time now and have seen people get hired in at very junior-level positions rise to management and product owner roles by identifying problems and working their butts off to solve them.
Cons
As with any company, dominant personalities tend to control most of the decisions that get made at Riot. Once people have established you as someone "junior," on your team, it's difficult to reverse that perception in a way that allows for increased career opportunities. A mantra of many successful companies is to "hire well, fire quickly." Riot has the first part down, but is slow to fire unproductive/unmotivated employees, especially since many Rioters tend to be friends with each other to some degree on their respective teams. It often comes down to 2-3 superstars carrying the load for the team while others are content to do the bare minimum and skate by.