Sunset Overdrive is getting a TON of attention for the exact same reasons as Splatoon. In fact the only difference is that Sunset was announced last E3 and we saw gameplay a few weeks pre-e3, if we hadn't it would be getting talked about as much as Splatoon is.
In essence, Splatoon is getting attention because:
1) New IP
2) Played very well on the floor
3) Looks great
4) Original idea and presentation
5) Complete surprise
The fact that it's a Nintendo game just marks the quality people expect of it; if it was by another dev/ on another console, odds are it'd be a throwaway 'kid' game which sucked.