As a person who is really into atoms and compare and set as a concurrency primitive, it would be nice to be able to do a compare and set on mutable fields in a deftype without incurring the overhead (more objects taking more memory, and more pointers to chase) of either a java AtomicReference or a clojure atom.
The jvm has several mechanisms that allow you to directly cas on a field (AtomicReferenceFieldUpdater , VarHandles. maybe others). It seems like deftype could make such a thing available on request.
It is currently possible to use AtomicReferenceFieldUpdater with deftype, but is somewhat convoluted due to volatile fields being private, and the lack of static initializers.