Despite several researchers reporting evidence that 3D Virtual Worlds can be used to effectively support educational processes in recent years, the integration of this technology in real learning processes is not as commonplace as in other educational technologies. Instructional designers have to balance the cost associated with the development of these virtual environments with the expected outcomes reported by the use of the new technology, but for some types of learning processes those outcomes are not always easily predicted. In this document the authors experience using 3D Virtual Worlds is summarized with the aim of getting a deeper understanding of their potential pedagogical use when supporting two different types of learning activities commonly included on a course: direct instruction, which exploits the social dimension of the technology, and individual learning activities in which that feature is not used. Based on those experiences a set of guidelines for designing 3D virtual world learning environments is proposed.