Two words: Company Culture. Everyone is always happy and laughing. People get along with one another well and you really feel a sense of teamwork working there. Senior management makes an effort to get to know everyone and participate in the summer outings. HR is approachable and friendly. Offices are beautiful. Location is great. Internships are great.
Cons
can be clique-y. Meaning departments tend to hang out and stick together.
There are few other jobs where the connection between the work that you do and the benefit to other people (or society in general) is so apparent. The employees here are top-notch, and while they work hard, they also know how to play hard too. Management has a great deal of respect for the employees, and we're treated very well. Salary's great, flexible time off policy, company paid happy hour once a week, stocked office refrigerator...
Cons
This is one step up from a startup, so there's still significant growing pains. Plenty of work to be done, and nowhere near enough time to finish it all. HR/IT are located in Beverly, MA, so here in Boulder we sometimes feel a bit like the ignored stepchild.