Pros
I've been at Riot for almost a year now, and the experience has been very rewarding. My favorite part about working here is the autonomy you get. Other people at the company trust that you know your stuff and won't micromanage or question everything that you do. You are judged by your results (and people will judge them!). This trust is a result of an admittedly grueling interview process in most disciplines that tries to ensure that you are player-focused (you'll use your autonomy to make the right thing for the players) and that you are a master of your discipline (you'll use your autonomy to make things the right way). Another aspect of Riot that I love is how open to feedback individuals are and the company as a whole. Candid feedback is encouraged, solicited, common and responded to. A recent test of this was when there was a change in payroll that was announced ahead of time, elicited a ton of negative feedback and was promptly cancelled because of the feedback. Every company I have ever worked for in the past would have not even asked ahead of time, let alone changed their minds. Finally, it is just amazing to be surrounded by the quality of people here at Riot. Not only is everyone brilliant, they are generally humble and willing to help with anything.
Cons
Riot is definitely experiencing growing pains with a dearth of strong leadership in many areas. This has lead to junior Rioters without sufficient mentoring. The interview process is very long and often confusing to candidates. It is for a good reason, but we can do a much better job cleaning up the process and communicating more clearly with candidates.