Jenkins 2 now requires Servlet 3.1 to run. If you're not using the embedded Winstone-Jetty, make sure your container supports Servlet 3.1 (e.g. Tomcat 8) before upgrading.
Jenkins 2 no longer supports AJP with the embedded Winstone-Jetty container, so if you're using Jenkins with a reverse proxy, please make sure it uses HTTP before upgrading.