If the Zelda franchise could be said to have laid the foundation for medieval-paradigm fantasy adventure gameplay, The Elder Scrolls franchise expanded upon it to factor in this fascinating concept:
Do whatever you want.
Want to be the hero and play through the main storyline? Sure, go for it.
...Want to be completely ignore the storyline altogether? That's fine too.
Want to be the most powerful two-handed weapon wielding warrior ever known? Or would you rather rely purely on powerful magic spells?
Want to simply explore all the dungeons and see what treasure you find? Want to steal, sneak, and pick locks for the entire game? Want to stop and read the literally hundreds of small books and pamphlets strewn throughout the land? Or would you rather just be evil, and kill everyone in your sight?
Play the game however you want. The world is open to your choices. It offers the vast size and complexity of an MMORPG, without the annoyance (to yours truly, anyway) of other players. (As an aside, a good friend of mine made it his goal to steal every last item in Morrowind, the third entry in this series.)
As open and expansive a world as the Elder Scrolls franchise brings to the table, it would be a fair critique of this game's predecessors to say that they did not offer enough guidance to the player when first stepping into the quest.
I personally had a difficult time progressing through the main story in Morrowind without a printed in-depth guide by my side. The storyline took some time to bring the excitement, and there was no mechanism in place to make travelling less time consuming. Furthermore, the game would not warn you when you were creating a character with poor stat combinations who was destined to be an enemy's doormat.
Oblivion introduces a very handy fast-travel system, allowing you to instantly return to any locale that you had previously visited (automatically advancing the in-game time to account for your movement). The game also revamps the starting character creation process, allowing for a much easier selection of ready-made stats (while simultaneously offering an even greater degree of customization, especially in appearance).
And certainly, no one can accuse Oblvion of taking too long to get to the action and adventure, as it throws you right into a dungeon-crawl, with the great Patrick Stewart (of Star Trek: The Next Generation and the X-Men movies) playing the Emperor, who tells you of the coming evil from the Oblivion realm.
For good measure, Oblivion also adds horseback riding to the mix, a first for the franchise.
Oblivion was one of only two games that I ever earned ALL the Xbox Live Achievements for (requiring one to complete all of the main game, all of the side quests, and all of the Shivering Isles expansion). And I loved every minute of it.
From what time I have spent with Oblivion's hotly anticipated sequel, Skyrim, it is apparent that the magic of the world of Tamriel shall live on for many more games to come...