Cult Classic: Although a hit when it was first released, Morrowind was eventually eclipsed by its sequels in popular eyes (arriving at Stage 6a of the Fandom Life Cycle). Nonetheless, it is still actively played and modded by a fiercely dedicated and very militant fandom core to this day.
Killer App: This game was one for the Xbox for those fans who didn't buy one for Halo, being one of the first then-modern Western RPGs available on a console.
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 has been the longest between games in the main series to date. (Though by now the gap between Skyrim and The Elder Scrolls VI is going to be even wider.)
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...
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.
In very early planning, this game was almost called "Tribunal" and would have taken place in the Summerset Isles, homeland of the Altmer.
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.)