First, I'm glad to hear you like our paired-pages presentation format.
Second, the biggest change with Java 9 is project Jigsaw, which is enabled by the new module system. This divides the Java API into a series of modules, so that you can choose which modules you want to include in your application. This should lead to performance improvements for deployed applications. Also, it makes it possible for you to write Java code that's truly modular, even more modular than when you use packages, classes, etc. I think this is easily as significant as lambdas or generics, but I think it's probably most significant for the people who develop the Java API as it should make it easier for them to add new features to future versions of Java, now that the Java API has been retrofitted to be truly modular. Like most new Java features, you can ignore it if you want. In that case, you use the old system of packages, classes, etc. to create code that's mostly modular.
I hope this helps! Thanks!