I still struggle sometimes, coming from a spring framework mindset, figuring out how to do some stuff. Though I love the @Provides support in trunk of Guice. Here's hoping 2.0 is out soon! :)
I've experimented recently adding support for ConstructorInterceptors into guice so you can add custom lifecycles like @PostContruct from JSR250/EJB3 or Spring's InitializingBean etc.
I've also created a patch to support closing of singletons such as via @PreDestroy or spring's DisposableBean.
Slightly more wacky is a patch adding support for custom injection point annotations which allows you to easily support things like