Make sure your co-workers are people you want to spend time with

Most people spend more time at work than anywhere else. So, in addition to feeling like your contributions are meaningful, be sure that you enjoy spending time with your co-workers. This may sound obvious, but it's easy to convince yourself to take a job based on compensation, prestige, or title / responsibilities.

Are your co-workers wired the same way as you? This doesn't mean that you'll always agree, but it does mean that you share core values and instincts. For example, if you're joining a startup, are you co-workers inclined to act even if they'll need to course correct, or are they used to working at larger companies where they can delegate and take more time to deliberate? Just because someone was successful in large company environments, it doesn't mean that he/she'll thrive at a startup. And of course vice versa!