A hypermedia application offers its users much freedom to navigate through a large hyperspace. Adaptive hypermedia (AH) offers personalized content, presentation, and navigation support. Many adaptive hypermedia systems (AHS) are tightly integrated with one specific application and/or use a limited number of techniques and methods. This makes it difficult to capture all of them in one generic model. In this paper we examine adaptation questions stated in the very beginning of the AH era and elaborate on their recent interpretations. We will reconsider design issues for application independent generic AHS, review open questions of system extensibility introduced in adjacent research fields and try to come up with an up-to-date taxonomy of adaptation techniques and an extensive set of requirements for a new adaptive system reference model or architecture, to be developed in the future.