So far in my exploration of Java 8 I have encountered a couple of "why'd they do it like that?" discussions.
One particularly contentious new class is java.util.Optional.
The reasoning behind not implementing Serializable boils down to discouraging developers from misusing the concept. It's not intended to be stored as a field value, but rather to act as a temporary representation when returning a potentially null value from a method.
See the relevant JDK 8 Developers mailing list discussion for the range of perspectives.
I suspect my current team and I may have already misused Optional - time for some refactoring...
Unexciting update: I managed to find 0 offensive usages of Optional in the current project's codebase.