Also note that many languages that encourage immutable data also have first-class functions with closures, which are also a very powerful tool for encapsulation

Also note that many languages that encourage immutable data also have first-class functions with closures, which are also a very powerful tool for encapsulation and abstraction. I think lambdas more than immutability help one write clean abstractions without traditional OO-objects. See the "Lambda the Ultimate" series of papers for details.