Pros
Loads of responsibility and total trust from early on
Opportunity to work with loads of interesting, knowledgeable people.
Exposure to a range of practices and skills (UX, Ops, HR, Coaching, Sales, Marketing)
Learning budget to spend as you like
Flexibility in how and when I work
Great balance between work and fun in my opinion
Cons
The usual start-up challenges ... uncertainty, constant change so sometimes processes aren't ideal but it's down to you to improve anything you want to improve!