Fun, fast moving and interesting!
Pros
Great people, great culture (and big focus on keeping a healthy culture), interesting developing field (gaming, mobile and AR/geo). Lots of interesting work (sometimes too much of it, see below). Great, nice and very talented folks who focus on getting things done. Not a lot of bureaucracy or time/soul-sucking meetings. You really feel like you have an impact/ are moving the needle/ are building something from nothing. Healthy mature & adult culture, ie, good mix of tech veterans and youth. A "startup run by adults". Most folks are pretty hungry and excited to make things happen. Already had some success, and good recent investment, so have lots of runway to execute on a good plan.
Cons
Sometimes too much work to do, but the company is at that exciting phase of its development, will not last forever. And they are staffing up to handle the work. Some of the old timers don't try as hard as one would like, but that is the minority, most folks are focused on execution/ getting stuff done.