Here is a list of the really cool features of Sling:

• totally RESTful: resources = resource tree ~= JCR + default HTTP methods already implemented
• thin server architecture: no need for complicated object models to get from URL to relational db storage
• resource thinking everywhere: server code can access resources (eg. include them) freely in the same way as requests are handled
• if JCR is not enough, the resource tree can be extended by code that resolves specific resources in a completely different way
• full filter pipeline, both for pre-request and post-request handling (and in both scripts and Java)
• support for many scripting languages (Javascript, Ruby, Python, etc.) - for quick development
• support for Java servlets - for IDE support
• more uptime through OSGi - updates at runtime!
• controlled software lifecycle management through versionized OSGi bundles
• import/export of resource data via JSON natively - great for AJAX apps