Multi-Platform: For both Xbox and PC, making it the first game in the series to be multi-platform. This allowed the game to get into the hands of a wider audience, helping make it a Breakthrough Hit for Bethesda and introduce a massive Newbie Boom.
Sequel Gap: The six-year gap between Daggerfall and Morrowind is the longest between games in the main series to date.
Urban Legend of Zelda: There were supposedly a number of ways to join Dagoth Ur and/or the Sixth House, most of which were debunked rather quickly. As you can see in the below entry, this one was almost a real part of the game...
What Could Have Been: The blight was to originally be an expanding threat, and Dagoth Ur was originally to be joinable. Technical limitations axed the former, and time limitations axed the latter.
Word of Dante/Word of Saint Paul: Some of the game developers have posted "obscure texts" on the official forums regarding a number of in-game topics and characters. They are generally regarded by the fandom as canon. Michael Kirkbride, in particular, wrote a number of them which fill in gaps around The Tribunal, Vivec, and the Dwemer. (Kirkbride is also the author of many of the in-game books, including most-famously the 36 Lessons of Vivec series.)