Doesn't look like they've put together an official changelog yet, but the major changes off the top of my head would be jQuery, Coffeescript, sass, sprockets, css sprites, flushing & deprecation of rails 2 finders.
2. The file_field automatically adds :multipart => true to the enclosing form.
3. Newer Migrations - There is a "change" method now which can be used to write only the "up" part of the migration and Rails will automatically reverse when a migration is rolled back.
