I don't understand the hype either, also that it stood in the way of a potential Super Mario Sunshine "2"
(since it was originally planned as a Mario game)
It's a colorful shooter with some fun looking mechanics where I'm 100% guaranteed not to have to listen to little foul mouthed squeekers scream about how they screwed my mum.
One of the reasons why Splatoon is considerably hyped is mostly attributed to how much it reinvigorates the tired online shooting genre and made it accessible to the much younger audience, something that the previous decade sorely lacked.
For quite a while, most online shooting games are designed with teenagers and adults in mind. I won't deny that my point of view is old-fashioned, but that's pretty much what drags the online shooters down in the drain, especially during the last decade.