Pros
Great colleagues who are smart and talented and driven. Lots of amazing initiatives to improve what we're doing, inside and out. Benefits reflect the culture and values. Emphasis - in office culture and in our work - on families and enjoying the outdoors. Quirky old office on Beacon Hill (centrally located, homey feel). Though not everyone has felt this way, I feel I have both responsibility and agency in my role - this has not always been the case in previous positions.
Cons
Busy times get really busy with a small staff. Be prepared to wear many hats, share space and tough it through leaner times. This seems to be the case for many nonprofits, but it's worth mentioning. Quirky old office on Beacon Hill (it's three brownstones joined by a web of stairs - there are downsides). We're never outdoors as much as we'd like to be. It can be hard to affect change.