The sense of accomplishment and learning new things. This being my first job ever, I learn something new everyday at work, and that is the best feeling ever!
The coworkers and managers are all very friendly and easygoing and that makes it easy to get any kind of work done while having fun at the same time.
Everyone on the team has a sense of responsibility and accountability towards their job. The level of flexibility and the passion everyone around you has when working certainly does make you want to go the extra mile.
My to-be manager (back when I was still interviewing) was constantly in touch with me throughout the process and assured me that they're doing everything they can to get the decision finalized as soon as possible. I don't think you see that kind of effort a lot out there in the job market.