Pros
Much of the job satisfaction derives from the fact that it's a mission-driven organization whose goal is to reduce poverty and raise the standard and quality of living in the poorest countries of the world. And, it is a stimulating intellectual environment - the multitude of issues that the institution works with alone makes it an interesting place to be. It's full of very smart people, very well educated, most of whom are genuinely engaged and care very much about fulfilling the organization's mission. Many, many opportunities to learn, super interesting colleagues, travel, a truly global environment, excellent benefits - maybe not as good as in the past but still in the top tier, including leave policies that model the Europeans more than the Americans.
Cons
One's experience here can vary considerably depending on who you're working with. I had almost without exception excellent managers who genuinely, actively supported my professional development. But I have colleagues and friends who didn't have such good fortune. Additionally, it relies excessively on short-term contractors and consultants, it is very difficult (or at least it was, at the time I left) to get a permanent, full-time position. There's also no getting around the fact that it's a bureaucracy, which all large organizations inevitably are, but this one perhaps more so than some corporations because it's literally political: the board of Executive Directors consists of representatives of the member countries, who meet not just once a quarter like many if not most corporate Boards, but rather twice a week in full committee. Geopolitics inevitably drives a fair amount of what happens here, but that's also partly why it's such an interesting and stimulating place to work.