Addition to my previous post: as an example of the negative role of Dan in end-time scenarii, read the Testament of Dan, especially chapter 5: http://www.ccel.org/ccel/schaff/anf08.iii.ix.html. Such ideas were carried over into early Christianity since both Irenaeus (Adversus Haereses, V,30,2) and Hippolytus (De Antichristo, 14) echo the tradition that the Antichrist would come from the tribe of Dan.
Btw, the Testament of Dan may offer another possible clue to the inclusion of Joseph -- along with (his son) Manasseh instead of (his other son) Ephraim -- in the Revelation list, in addition to the frequent criticism of Ephraim in the "prophetic" books (which in Jewish tradition includes what we term "historical" books like Judges and Samuel-Kings): Dan's "sin" is mostly against Joseph (midrashic development on Genesis 37ff); Joseph is vindicated and glorified while Dan is condemned.
A more general reflection on snowbird's remark:
According to the angel who presented it to John, we are supposed to be happy about reading it!
This imo illustrates the basic problem with Revelation (as well as with most Bible texts, but much more clearly with Revelation). It was not written to us, or for us. The original audience could understand its "symbols" quite easily because they were part of their political, religious and cultural environment. They were not "scholars," they didn't need any special knowledge to "uncipher the code," but average Christian communities of Asia Minor, as the reading instruction you refer to suggests (1:3) : as was usual back then, in a context of limited literacy, one person reads the book aloud for others to hear. This target audience has disappeared long ago, but the book remained and was eventually (and painfully) canonised. Nowwe need to delve into several specialised fields of scholarship (late Judaism, early Christianity, ancient history of the Eastern Roman empire) to understand in part what it was about. But the temptation is strong for every generation of Bible readers to shortcut such scholarship and simply assume that the book must be directly meaningful to them. For a 16th-century Protestant reader it was naturally about Reformation and Papacy; to a 21st-century American reader, it has to relate with 21st-century America.