Well, the game has 300 hours of content, 150 dungeons (hey I like dungeon delving, so sue me) and a 40 hour story (iirc, might have been 30).
So it's big, fine.
It's also pretty, the screenshots (actual, not rendered) consistently show it to be an improved Oblivion in looks, much improved. Especially people.
It's big and it's pretty. Cool.
It improves on the gameplay aspects of Oblivion, retaining the logical (and beautiful) use it to improve it skill levelling while fixing the horrific points allocation at level up and introducing perks (or a similar thing) in lieu of set bonuses for reaching a certain point in a skill.
Throw in the entire epic thing, the rich lore and backstory, the pretty powerful story and a very, VERY large amount of customisation and roleplaying opportunity and you have a wonderful game.
It works for those who are fans of the series, those who just like RPG's and those who just want to run around swinging their sword at skeletons and dragons.
That's why it's a big deal: It's a fantastic game (or, will be).