• What's new?
• Has a more consistent API.
• Has a more flexible and simpler menu system.
• Has a more flexible and simpler JS and CSS include system.
• Has a more consistent coding style.
• Has a more robust river system.
• Has a more robust API for annotations and metadata.
• Has a more logical and easier to use views structure
• Has significantly improved markup and CSS.
• Has an improved forms system, including sticky forms.
• A JS engine that allows you to add Ajax easily.
• Has a simpler and more intuitive admin area.

There is a File called "CHANGES.txt" that comes with every Elgg release where-in all Changes are described ;-)