Content repository: at its core, an LMS is a place to keep stuff and distribute it to students. In a sense, the LMS is the descendant of the "ditto machine" via the photocopier.
Administrative Tool: An LMS allows an instructor to maintain an accurate course roster and record information about students in the course, e.g. attendance and grades.
Single Point of Contact: To the extent that all or most of a campus uses a common LMS, students can expect to go one place and find everything they need for all their classes.
Faculty/Student Communication Tool: A closed (like a classroom) environment used for faculty-to-student communication (often), student-to-faculty communication (sometimes) and student-to-student communication (rarely).
Learning Tool Portal: An LMS is often used as a way to authenticate students for the use of learning tools such as blogs, video servers, proprietary data bases, textbook related materials, etc. The IMS Learning Tools Interoperability (LTI) specification makes this easier although it still remains a challenge.
A key feature, driven directly by the classroom metaphor, is that this is a closed space. You're in the class, or you're not, and typically if you're not in the class you see nothing - although some LMS's have begun to develop more granular access models. Certainly there are advantages - particularly related to student privacy and copyright management - but also significant limitations imposed by the closed classroom model. In later postings I'll focus on the limitations.
Do you agree? Have I missed something important that isn't obviously related to one of these five attributes? Please let me know!