Being Dovahkiin is part of the PREMISE, just like the prophecy and all else revealed. What happens then on, is obvioulsy NOT gonna be revealed.
I'm well aware, but the concern for me is that knowing you're the Dovahkiin early on could severely reduce the variety in the main quest. A major portion of Morrowind's main quest involved learning about the prophecies, and confrontations with different factions and their efforts to stop or capitalise upon the Nerevarine's return. Sure, if you compartmentalize the quests they were mostly all random errands, but I would hate it if half of Skyrim's main quest consisted of "pass the trials, collect the magic artifacts" scenarios like Oblivion's did.
There should be some degree of study and an almost mystery-like element to the story, which was done fairly well in Morrowind as you read about the different prophecies and had to interpret them (or at least sit through NPCs' explanations about them) in order to proceed. Anybody remember the quest
Mehra Milo and the Lost Prophecies? Not only was that easily (by my estimates) one of the most tense moments in the game's plot, but travelling to Holamayan and meeting with other individuals who gradually helped to guide you along your quest was a very well-executed, almost cinematic point in the main quest for me.
Oblivion had its great moments to, especially virtually all encounters with Mankar Camoran and the Mythic Dawn. But a lot of it also seemed pretty irrelevant and detached from the world itself and your personal character. "I'm going to this dungeon to grab this item because Martin said so." I guess in Morrowind lots of your actions (Hortator and Nerevarine) were dictated by prophecy as well, but at least they were a bit more intertwined with the game's factions.
Hopefully Skyrim's main quest won't be toned down due to additional effort being spent on factions and Radiant Story. Though hardly the most important part of the game, a great main quest can serve as a means to slowly introduce your character to more of the world's lore and characters in a natural faction.