Oblivion marked a turning point in The Elder Scrolls series by upping the graphics level and putting more into the game's non-player characters. There was also more of a focus on the main story than previous joints in the series. Bethseda also raised the bar with schedule driven characters. The next time you're in a village and a character tells you to come back at 9PM, thank Oblivion for that.
The Elder Scrolls: Oblivion