I tried different example snippets for gridfs from http://mongodb.github.io/node-mongodb-native/2.0/tutorials/streams/ and all results in following error. Let me know if you can reproduce. Particularly using example 'Streaming a File into GridFS':

/Users/athli/Devel/gridFS/node_modules/mongodb/lib/mongo_client.js:386
throw err
^
TypeError: Cannot read property 'w' of undefined
at _getWriteConcern (/Users/athli/Devel/gridFS/node_modules/mongodb/lib/gridfs/grid_store.js:1301:22)
at new GridStore (/Users/athli/Devel/gridFS/node_modules/mongodb/lib/gridfs/grid_store.js:124:23)
at /Users/athli/Devel/gridFS/index.js:11:21
at /Users/athli/Devel/gridFS/node_modules/mongodb/lib/mongo_client.js:383:11
at process._tickCallback (node.js:419:13)