You can now make copies in serial mode. That means that the copies are created after each other instead of parallel. This can be a good idea when you update a live resource. The resources will go down and up after each other.
A sample of this:
{ "apiVersion": "2015-01-01", "name": "[concat('nestedDeployment',copyIndex())]", "type": "Microsoft.Resources/deployments", "copy": { "name": "myCopySet", "count": 4, "mode": "serial", "batchSize": 2 },
The mode and batchSize are new. Mode can be serial or parallel. The batchSize configures how many objects are created at the same time in serial mode.