I think Splatoon looks like it could be a great competitive game with all the subtle amounts of depth to it. The reason it's getting as much attention as it is is because it's a new Nintendo IP. Nintendo rarely creates new IPs outside of spinoff games so when a new IP does get created, fans think it's the best thing ever. Like I stated about fans hyping up certain characters like Pit(who was dead) or Rosalina. They have all that praise for new characters or the rare new IP.
It's a Nintendo first party exclusive, and as always, anything Nintendo makes is the best thing ever according to Nintendo fans, regardless of whether they like the genre or not. I'm sure a lot of fans who hate shooters are throwing endless amounts of praise at the game because it's from Nintendo.