Cocoon is quite different from most other web development platforms. It may be not as suitable for sites with complex business logic or heavy interface, but thanks to its XML/XSLT focus it is much more convenient for document-centric web development and publishing.
The difference between Cocoon and other web development platforms is, in a sense, similar to the difference between XSLT and the traditional programming languages: being less universal, it more powerful and convenient in its specific area.