The big Java 8 feature is of course lambdas with all its ramifications (method references, default and static methods in interfaces, streams). That's about 1/3 of my book.
There is a new date/time API that is very nice--you'll never want to use GregorianCalendar again.
If you care about this, there is a high-quality embeddable JavaScript implementation that interoperates with Java. Just last week, in a project on which I am working, we needed some degree of end-user customizability and were about to design some configuration language. Then we realized we could just let the users specify the customizations in JavaScript.
If you still do rich GUI clients, you will want to know about JavaFX. It comes automatically bundled with Java 8, as Swing is now in "maintenance mode".
There are concurrency enhancements, mostly for advanced users. For example, it is now easy to make threadsafe maps of counters. (This used to be surprisingly painful.)
And of course, there are a bunch of minor changes (easy
string joining, a mod method with sane behavior for negative inputs, official support for Base64 encoding, etc.) I searched through the source tree and collected the useful nuggets so you wouldn't have to.
Cheers,
Cay