For those of us more dictionary oriented, there is https://pypi.python.org/pypi/voluptuous (which is OK for the most part, as long as you are only trying to do validation, and nothing too crazy)
Schematics has a very useful feature in "roles" - i.e. a good way of hiding certain fields in certain situations (e.g. admin views vs self vs other vs anonymous). Does marshmallow have something similar?
I've used it for parsing various network protocols to good effect.