In AEM 6.1, the default repository based ClassLoader was replaced with the new FSClassLoader (File System ClassLoader). This post talks about how this affects developers as well as introducing a new OSGi console to help developers debug issues in JSP development.
AEM 6 ships with a new repository, CRX3 which is based on Apache Oak, a subproject of the Apache Jackrabbit project. This repository promises significant performance increases over CRX2, however it does require a bit more work to setup. Unlike CRX2, CRX3 requires you to explicitly declare which fields will be indexed.