It requires more work to involve the public and requires additional work to meet requests for information, assistance and substantiation of conclusions. It means communications not solely with people who know something about planning, but also with people who are not as aware of the issues. All of this adds to the expense of the planning process. Apathy is probable, as well. In all likelihood, there will not be a groundswell of public interest in the task of creating a new plan.
Planning often receives little attention because issues are long-term as opposed to immediate.
Citizen consensus is hard to come by. There are very few subjects on which the general citizenry ever has a consensus opinion. This can become a major pitfall if a community is severely divided. Large numbers of participants can come away from input meetings with the feeling that they were not listened to. Finally, special interests may try to dominate.