Pros
Twitch is an awesome product. The company is working on cool projects and employs pretty strong engineers. It is a great place to learn engineering best practices and do something impactful. You will shortly become an expert in AWS, too. Work and fun often go hand in hand: say, watching Twitch during the business hours and playing video games after hours is quite common. The corporate culture is centered around ownership and KPIs, which is both good and bad. If you develop a service that does not correlate directly with the KPIs, your next manager may tell you to find a new owner or decommission it.
Cons
My biggest complaint is about compensation policy, which Twitch inherits from Amazon. During the last 2 years of the 4-year contract the annual compensation consists of the base salary (which is rumored to have a cap at $185K) and 40% of the RSU grant. Given impressive performance of AMZN stock, this sounds lucrative. In reality once compensation moves into the bracket for the next level, the management is not happy about it. Those who get promoted see only little changes in their compensation and the new hires at the same level get a much better pay. On the other hand, if you are not promoted, chances are that a cheaper new hire will replace you right before your next RSU vesting. So IMO it does not really make sense to stay beyond the 2-year anniversary.