I think groundbreaking in the sense of visuals and scale not gameplay and story. Oblivion, fallout 3 and skyrim were impressive looking games at the time they came out which was a lot of the appeal at the time. Well it was for me, now Skyrim can run on a phone its not so novel and exciting. Although being able to play the games on a portable device has its own novelty.

There's never been anything groundbreaking about it, it managed to be just the right amount of dumbed down for console audiences while also solidifying the core "quest>dungeon>loot>sell loot >quest" loop that turns it into the addictive hole that people fell into when they played it for the first time.

edit: also helps that it's not fundamentally busted and ugly like Oblivion (which really doesn't even have much to offer over Skyrim besides its goofy and inept charm)