For me, it is mostly the movement. I don't really like shooters very much because moving around is boring, generally. I like some Halo, TF2 and Unreal Tournament sometimes because movement is unlike it is in "modern" shooters like Call of Duty, Battlefield, etc. Splatoon's movement looks exciting and tricky. Lots of potential there.
Plus, there is always something to do in Splatoon. When you're walking back to the front lines, you can at least shoot some ink at some un-inked places. That is, if you don't super jump right back into the action. It seems like there won't be a dull moment in Splatoon, except MAYBE right when the game starts and you ink your base while walking to the battleground.