"Never lose sight of why software is being developed: to satisfy real needs, to solve real problems. The only way to solve real needs is to communicate with those who have the needs. The customer or user is the most important person involved with your project."
It may feel like it is easier to develop in the sweet silence of a vacuum but will the finished software be something that the customer likes or even finds useful? If your customer or product manager is not easily accessible then designate some people on your team to be advocates for your customer. Ask them to imagine being the customer and get their feedback. If possible, have them use the software as if they were the customer and document the good and bad points.
