RAP applications are basically RCP applications that run in a servlet container environment. Consequently, you should have a working understanding of the underlying technologies to develop RAP applications. Those technologies include SWT, RCP, Equinox OSGi, and Servlet technology.
Many questions and problems that occur while developing RAP applications are not particularly related to RAP, but to the technologies below. As an example, layout issues are usually SWT-related, dependencies to third-party libraries are handled on the OSGi level, and questions regarding session handling are related to the servlet container.