[*]The main quest line is clearly much, much shorter in Skyrim.
[*]The Mages guild is much shorter, though in Oblivion it was quite redundant with all of the chapters.
[*]Thieves Guild is a push since I don't remember it in Oblivion besides the epic final heist where you steal the Elder Scroll.
[*]Dark Brotherhood is probably slightly shorter.
[/list]You could say that the stories and general quality in Skyrim is much better. But when just looking at how long it took you to complete them, what were your results?