It would be really great to be able to specify that a collection is sharded on given tags.
Usage : datacenter awereness => this collection should be sharded on a given datacenter only.

Without this feature, performant multi-datacenter clusters are not really possible since it is impossible to control where the data will be moved.

It will raise an issue with the difference of tags between members of a replicaset (if using sharding and replicasets).