Whether Cyrodiil is 1.5 times or 6 times as big as the 1-50 levelling areas - it still means that most of the endgame content will be in a "world PvP" area. Unless you'll have flying mounts or insta-travel "waypoints" that get you to your factions "safe area" or fortress/outpost within Cyrodiil that means that dungeon runners will have to travel through more or less dangerous areas with the chance of getting killed by other players. Be it individuals or roaming zergs or groups that camp certain passages or dungeon entrances to prevent players from opposing factions from entering.
Depending on how big the risk is, I think a lot of MMO players won't like that at all. It will cost TESO a lot of "carebearish" players.
TESO seems to rather cater former hardcore Darkfall/DAoC players than TES players and PvE oriented type of players.