Testing with GeoServer 2.9.1 against MongoDB 3.2.8 – GeoServer is unable to connect to or create document stores. Connections to the database itself succeed however no layers are ever shown, despite being properly configured and having the requisite spatial index on them.
Steps to reproduce:
1) run a recent version of mongo "docker run -p 27017:27017 -d mongo"
2) create sample document collection and sample document with simple point field
3) create spatial index
4) Install Mongo Extension in GeoServer and attempt to connect to mongo
5) Data store will be created however no layers will be shown