This question is usually the first question everyone has when a new release comes out. Oracle now has a neat tool to help answer that question. Its called the Cumulative Feature Overview (CFO) tool and provides concise descriptions of new and enhanced solutions and functionality that have become available between your current release (8.4 forward) and a specific target release, now including 9.1. The tool is available from both the FMS as well as SCM 9.1 Documentation Home page. The FMS page is located here and the SCM page is located here.